Overview of Gis

7/23/2019 Overview of Gis

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OVERVIEW OF GIS


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INTRODUCTION

Geographic Information System (GIS) is a computer based information system

used to digitally represent and analyse the geographic features present on the Earth'

surface and the events (non-spatial attributes linked to the geography under study) that

taking place on it. The meaning to represent digitally is to convert analog (smooth

line) into a digital form.

"Every object present on the Earth can be geo-referenced", is the fundamental

key of associating any database to GIS. Here, term 'database' is a collection of

information about things and their relationship to each other, and 'geo-referencing'

refers to the location of a layer or coverage in space defined by the co-ordinate

referencing system.

Work on GIS began in late 1950s, but first GIS software came only in late

1970s from the lab of the ESRI. Canada was the pioneer in the development of GIS as

a result of innovations dating back to early 1960s. Much of the credit for the early

development of GIS goes to Roger Tomilson. Evolution of GIS has transformed and

revolutionized the ways in which planners, engineers, managers etc. conduct the

database management and analysis.

DEFINING GIS

A GIS is an information system designed to work with data referenced by

spatial / geographical coordinates. In other words, GIS is both a database system with

specific capabilities for spatially referenced data as well as a set of operations forworking with the data. It may also be considered as a higher order map.

GIS technology integrates common database operations such as query and

statistical analysis with the unique visualization and geographic analysis benefits

offered by maps. These abilities distinguish GIS from other information systems and

make it valuable to a wide range of public and private enterprises for explaining

events, predicting outcomes, and planning strategies.


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A Geographic Information System is a computer based system which is used

to digitally reproduce and analyse the feature present on earth surface and the events

that take place on it. In the light of the fact that almost 70% of the data has

geographical reference as it's denominator, it becomes imperative to underline the

importance of a system which can represent the given data geographically.

A typical GIS can be understood by the help of various definitions given below:-

A geographic information system (GIS) is a computer-based tool for mapping

and analyzing things that exist and events that happen on Earth

Burrough in 1986 defined GIS as, "Set of tools for collecting, storing,

retrieving at will, transforming and displaying spatial data from the real world for a

particular set of purposes"

Arnoff in 1989 defines GIS as, "a computer based system that provides four

sets of capabilities to handle geo-referenced data :

data input

data management (data storage and retrieval)

manipulation and analysis

data output. "

Hence GIS is looked upon as a tool to assist in decision-making and management of

attributes that needs to be analysed spatially.

Answers GIS can give

Till now GIS has been described in two ways:

1. Through formal definitions, and

2. Through technology's ability to carry out spatial operations, linking data sets

together.

However there is another way to describe GIS by listing the type of questions the

technology can (or should be able to) answer. Location, Condition, Trends, patterns,

Modelling, Aspatial questions, Spatial questions. There are five type of questions that

a sophisticated GIS can answer:


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Location What is at.?

The first of these questions seeks to find out what exists at a particular location. A

location can be described in many ways, using, for example place name, post code, or

geographic reference such as longitude/latitude or x/y.

Condition Where is it.?

The second question is the converse of the first and requires spatial data to answer.

Instead of identifying what exists at a given location, one may wish to find location(s)

where certain conditions are satisfied (e.g., an unforested section of at-least 2000

square meters in size, within 100 meters of road, and with soils suitable for supporting

buildings)

Trends What has changed since..?

The third question might involve both the first two and seeks to find the differences

(e.g. in land use or elevation) over time.

Patterns What spatial patterns exists..?

This question is more sophisticated. One might ask this question to determine whether

landslides are mostly occurring near streams. It might be just as important to know

how many anomalies there are that do not fit the pattern and where they are located.

Modelling What if..?

"What if" questions are posed to determine what happens, for example, if a new

road is added to a network or if a toxic substance seeps into the local ground water

supply. Answering this type of question requires both geographic and other

information (as well as specific models). GIS permits spatial operation.

Aspatial Questions

"What's the average number of people working with GIS in each location?" is an

aspatial question - the answer to which does not require the stored value of latitude

and longitude; nor does it describe where the places are in relation with each other.

Spatial Questions


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" How many people work with GIS in the major centres of Delhi" OR " Which centres

lie within 10 Kms. of each other? ", OR " What is the shortest route passing through

all these centres". These are spatial questions that can only be answered using latitude

and longitude data and other information such as the radius of earth. Geographic

Information Systems can answer such questions.

Need of GIS?

Many professionals, such as foresters, urban planners, and geologists, have

recognized the importance of spatial dimensions in organising & analysing

information. Whether a discipline is concerned with the very practical aspects of

business, or is concerned with purely academic research, geographic information

system can introduce a perspective, which can provide valuable insights as

1. 70% of the information has geographic location as it's denominator making

spatial analysis an essential tool.

2.

Ability to assimilate divergent sources of data both spatial and non-spatial

(attribute data).

3. Visualization Impact

4.

Analytical Capability

5.

Sharing of Information

Factors Aiding the rise of GIS.

Revolution in Information Technology.

Computer Technology.

Remote Sensing.

Global Positioning System.

Communication Technology.

Rapidly declining cost of Computer Hardware, and at the same time,

exponential growth of operational speed of computers.

Enhanced functionality of software and their user-friendliness.

Visualizing impact of GIS corroborating the Chinese proverb "a picture is

worth a thousand words."

Geographical feature and data describing it are part of our everyday lives &

most of our everyday decisions are influenced by some facet of Geography.


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Philosophy of GIS

The proliferation of GIS is explained by its unique ability to assimilate data from

widely divergent sources, to analyse trends over time, and to spatially evaluate

impacts caused by development.

For an experienced analyst, GIS is an extension one's own analytical thinking. The

system has no in-built solutions for any spatial problems; it depends upon the analyst.

The importance of different factors of GIS in decreasing order is as under:

Spatial Analysis

Database

Software

Hardware

GIS involves complete understanding about patterns, space, and processes or

methodology needed to approach a problem. It is a tool acting as a means to attain

certain objective quickly and efficiently. Its applicability is realized when the user

fully understands the overall spatial concept under which a particular GIS is

established and analyses his specific application in the light of those established

parameters.

Before the GIS implementation is considered the objectives, both immediate and long

term, have to be considered. Since the effectiveness and efficiency (i.e. benefit against

cost) of the GIS will depend largely on the quality of initial field data captured,

organizational design has to be decided upon to maintain this data continuously. This

initial data capture is most important.

Advantages of GIS

The Geographic Information System has been an effective tool for implementation

and monitoring of municipal infrastructure. The use of GIS has been in vogue

primarily due to the advantage mentioned below:

Planning of project

Make better decisions


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Visual Analysis

Improve Organizational Integration

Planning Of Project

Advantage of GIS is often found in detailed planning of project having a large spatial

component, where analysis of the problem is a pre requisite at the start of the project.

Thematic maps generation is possible on one or more than one base maps, example:

the generation of a land use map on the basis of a soil composition, vegetation and

topography. The unique combination of certain features facilitates the creation of such

thematic maps. With the various modules within GIS it is possible to calculate

surface, length, width and distance.

Making Decisions

The adage "better information leads to better decisions" is as true for GIS as it is for

other information systems. A GIS, however, is not an automated decision making

system but a tool to query, analyze, and map data in support of the decision making

process. GIS technology has been used to assist in tasks such as presenting

information at planning inquiries, helping resolve territorial disputes, and siting

pylons in such a way as to minimize visual intrusion.

Visual Analysis

Digital Terrain Modeling (DTM) is an important utility of GIS. Using DTM/3D

modeling, landscape can be better visualized, leading to a better understanding of

certain relations in the landscape. Many relevant calculations, such as (potential) lakes

and water volumes, soil erosion volume (Example: landslides), quantities of earth to

be moved (channels, dams, roads, embankments, land leveling) and hydrologicalmodeling becomes easier.

Not only in the previously mentioned fields but also in the social sciences GIS can

prove extremely useful. Besides the process of formulating scenarios for an

Environmental Impact Assessment, GIS can be a valuable tool for sociologists to

analyze administrative data such as population distribution, market localization and

other related features.


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Improving Organizational Integration

Many organizations that have implemented a GIS have found that one of its main

benefits is improved management of their own organization and resources. Because

GIS has the ability to link data sets together by geography, it facilitates

interdepartmental information sharing and communication. By creating a shared

database one department can benefit from the work of another--data can be collected

once and used many times.

As communication increases among individuals and departments, redundancy is

reduced, productivity is enhanced, and overall organizational efficiency is improved.

Thus, in a utility company the customer and infrastructure databases can be integrated

so that when there is planned maintenance, affected people can be informed by

computer-generated letters.

Components of GIS

GIS constitutes of five key components:

Hardware

Software

Data

People

Method

Hardware

It consists of the computer system on which the GIS software will run. The choice of

hardware system range from 300MHz Personal Computers to Super Computers

having capability in Tera FLOPS. The computer forms the backbone of the GIS

hardware, which gets it's input through the Scanner or a digitizer board. Scanner

converts a picture into a digital image for further processing. The output of scanner

can be stored in many formats e.g. TIFF, BMP, JPG etc. A digitizer board is flat

board used for vectorisation of a given map objects. Printers and plotters are the most

common output devices for a GIS hardware setup.

Software


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GIS software provides the functions and tools needed to store, analyze, and display

geographic information. GIS softwares in use are MapInfo, ARC/Info, AutoCAD

Map, etc. The software available can be said to be application specific. When the low

cost GIS work is to be carried out desktop MapInfo is the suitable option. It is easy to

use and supports many GIS feature. If the user intends to carry out extensive analysis

on GIS, ARC/Info is the preferred option. For the people using AutoCAD and willing

to step into GIS, AutoCAD Map is a good option.

Data

Geographic data and related tabular data can be collected in-house or purchased from

a commercial data provider. The digital map forms the basic data input for GIS.

Tabular data related to the map objects can also be attached to the digital data. A GIS

will integrate spatial data with other data resources and can even use a DBMS, used

by most organization to maintain their data, to manage spatial data.

People

GIS users range from technical specialists who design and maintain the system to

those who use it to help them perform their everyday work. The people who useGIS

can be broadly classified into two classes. The CAD/GIS operator, whose work is to

vectorise the map objects. The use of this vectorised data to perform query, analysis

or any other work is the responsibility of a GIS engineer/user.

Method

And above all a successful GIS operates according to a well-designed plan and

business rules, which are the models and operating practices unique to each

organization. There are various techniques used for map creation and further usage for

any project. The map creation can either be automated raster to vector creator or it can

be manually vectorised using the scanned images. The source of these digital maps

can be either map prepared by any survey agency or satellite imagery.

GIS Applications

Computerized mapping and spatial analysis have been developed simultaneously in

several related fields. The present status would not have been achieved without close

interaction between various fields such as utility networks, cadastral mapping,


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topographic mapping, thematic cartography, surveying and photogrammetery remote

sensing, image processing, computer science, rural and urban planning, earth science,

and geography.

The GIS technology is rapidly becoming a standard tool for management of natural

resources. The effective use of large spatial data volumes is dependent upon the

existence of an efficient geographic handling and processing system to transform this

data into usable information.

The GIS technology is used to assist decision-makers by indicating various

alternatives in development and conservation planning and by modelling the potential

outcomes of a series of scenarios. It should be noted that any task begins and ends

with the real world. Data are collected about the real world. Of necessity, the product

is an abstraction; it is not possible (and not desired) to handle every last detail. After

the data are analysed, information is compiled for decision-makers. Based on this

information, actions are taken and plans implemented in the real world.

Major areas of application

Different streams of planning

Urban planning, housing, transportation planning architectural conservation, urban

design, landscape.

Street Network Based Application

It is an addressed matched application, vehicle routing and scheduling: location and

site selection and disaster planning.

Natural Resource Based Application Management and environmental impact analysis of wild and scenic recreational

resources, flood plain, wetlands, acquifers, forests, and wildlife.

View Shed Analysis

Hazardous or toxic factories siting and ground water modelling. Wild life habitat

study and migrational route planning.

Land Parcel Based

Zoning, sub-division plans review, land acquisition, environment impact analysis,

nature quality management and maintenance etc.


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Facilities Management

Can locate underground pipes and cables for maintenance, planning, tracking energy

use.

Application of GIS

GIS in agriculture

GIS is used in a variety of agricultural applications such as managing crop

yields, monitoring crop rotation techniques, and projecting soil loss forindividual farms or entire agricultural regions.

GIS in business

A GIS is a tool for managing business information of any kind according to

where it's located. You can keep track of where customers are, site businesses,

target marketing campaigns, optimize sales territories, and model retail

spending patterns. A GIS gives you that extra advantage to make you and your

company more competitive and successful.

A GIS enables you to better understand and evaluate your data by creating graphic

displays using information stored in your database. With a GIS, you can change the

display of your geographic data by changing the symbols, colors, or values in the

database tables.

GIS in electric/gas utilities

Cities and utilities use GIS every day to help them map and inventory systems,

track maintenance, monitor regulatory compliance, or model distribution

analysis, transformer analysis, and load analysis.

GIS in the environment

GIS is used every day to help protect the environment. As an environmental

professional, you can use GIS to produce maps, inventory species, measure

environmental impact, or trace pollutants. The environmental applications for


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GIS are almost endless.

GIS in forestry

Today, managing forests is becoming a more complex and demanding challenge.

With GIS, foresters can easily see the forest as an ecosystem and manage it

responsibly.

GIS in geology

Geologists use GIS every day in a wide variety of applications. You too can use

GIS to study geologic features, analyze soils and strata, assess seismic

information, or create 3-dimensional displays of geographic features.

GIS in hydrology

You can use GIS to study drainage systems, assess groundwater, and visualize

watersheds, and in many other hydrologic applications.

GIS in land use planning

People use GIS to help visualize and plan the land use needs of cities, regions, or

even national governments.

GIS in local government

People in local government use GIS every day to help them solve problems.

Often the data collected and used by one agency or department can be used by

another.

GIS in mapping

Mapping is an essential function of a GIS. People in a variety of professions are

using GIS to help others understand geographic data. You don't have to be a

skilled cartographer to make maps with a GIS.

GIS in the military

Military analysts and cartographers use GIS in a variety of applications such as

creating basemaps, assessing terrain, and aiding in tactical decisions.


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GIS in risk management

A GIS can help with risk management and analysis by showing you which areas

will be prone to natural or man-made disasters. Once identified, preventive

measures can be developed that deal with the different scenarios.

GIS in Site Planning

People around the world use GIS to help them locate sites for new facilities or

locate alternate sites for existing facilities.

GIS in transportation

GIS can be used to help you manage transportation infrastructure or help you

manage your logistical problems. Whether monitoring rail systems and road

conditions or finding the best way to deliver your goods or services, GIS can help

you.

GIS in the water/wastewater industry

People in the water/wastewater industry use GIS with the planning, engineering,

operations, maintenance, finance, and administration functions of their

water/wastewater networks.

Fundamentals of GIS

Mapping Concepts, Features & Properties

A map represents geographic features or other spatial phenomena by graphically

conveying information about locations and attributes. Locational information

describes the position of particular geographic features on the Earth's surface, as well

as the spatial relationship between features, such as the shortest path from a fire


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station to a library, the proximity of competing businesses, and so on. Attribute

information describes characteristics of the geographic features represented, such as

the feature type, its name or number and quantitative information such as its area or

length.

Thus the basic objective of mapping is to provide

descriptions of geographic phenomenon

spatial and non spatial information

map features like Point, Line, & Polygon.

Map FeaturesLocational information is usually represented by points for features such as wells and

telephone pole locations, lines for features such as streams, pipelines and contour

lines and areas for features such as lakes, counties and census tracts.

Point feature

A point feature represents as single location. It defines a map object too small to show

as a line or area feature. A special symbol of label usually depicts a point location.

Line feature

A line feature is a set of connected, ordered coordinates representing the linear shape

of a map object that may be too narrow to display as an area such as a road or feature

with no width such as a contour line.

Area feature

An area feature is a closed figure whose boundary encloses a homogeneous area, such

as a state country soil type or lake.

Map Characteristics

In addition to feature locations and their attributes, the other technical characteristics

that define maps and their use includes:

Map Scale

Map Accuracy


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Map Extent and

Data Base Extent

Scale

To show a portion of the Earth's surface on a map, the scale must be sufficiently

adjusted to cover the objective. Map scale or the extent of reduction is expressed as a

ratio. The unit on the left indicates distance on the map and the number on the right

indicates distance on the ground. The following three statements show the same scale.

1 inch = 2.000 feet => 1 inch = 24.000 inches => 1:24.000

The latter is known as a representative fraction (RF) because the amounts on either

side of the colon are equivalent: that is 1:24.000 means 1inch equals 24.000 inchesor1 foot equals 24.000 feet or 1 meter equals 24.000 meters and so on.

Map scale indicates how much the given area has been reduced. For the same size

map, features on a small-scale map (1:1,000,0000) will be smaller than those on a

large-scale map (1:1,200).

A map with less detail is said to be of a smaller scale than one with more detail.

Cartographers often divide scales into three different categories.

Small-scale maps have scales smaller than 1 : 1,000,000 and are used for maps of

wide areas where not much detail is required.

Medium-scale maps have scales between 1 : 75,000 and 1 : 1,000,000.

Large-scale maps have scales larger than 1 : 75,000. They are used in applications

where detailed map features are required.

So each scale represents a different tradeoff. With a small-scale map, you'll be able to

show a large area without much detail. On a large-scale map, you'll be able to show a

lot of detail but not for a large area. The small-scale map can show a large area

because it reduces the area so much that the large-scale map can only show a portion

of one street, but in such detail that you can see shapes of the houses.

To convert this statement to a representative fraction, the units of measure on both the


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sides being compared must be the same. For this example, both measurements will be

in meters.

To do this:

1. Convert 1.6 inches into meters

1.6 inches x 0.0254 meters/inch = 0.04 meters

2. Let us suppose that

0.04 units on the map = 10,000 units on the ground

Then, you can now state the scale as a representative fraction (RF): 0.04:10,000

Though it is a valid statement of scale, most cartographers may find it clumsy.

Traditionally, the first number in the representative fraction is made equal to 1:

0.04 / 0.04 = 1 units on the map = 10,000 / 0.04 units on the ground

1 unit on the map = 250,000 units on the ground

Scale in Digital Maps

With digital maps, the traditional concept of scale in terms of distance does not apply

because digital maps do not remain fixed in size. They can be displayed or plotted at

any possible magnification. Yet we still speak of the scale of a digital map.

In digital mapping, the term scale is used to indicate the scale of the materials from

which the map was made. For example, if a digital map is said to have a scale of

1:100,000, it was made from a 1:100,000-scale paper map.

However, a digital map's scale still allows you to make some educated guesses about

its contents because, generally, digital maps retain the same accuracy and

characteristics as their source maps. So it is still true that a large-scale digital map will

usually be more accurate and less general than a small-scale digital map.


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Because the display size of a computer-based map is not fixed, users are often

tempted to blow up maps to very large sizes. For example, a 1:100,000-scale map can

easily be plotted at a size of 1:24,000 or even 1:2,000-but it usually is not a good idea

to do so. It encourages the user to make measurements that the underlying data does

not support. You cannot measure positions to the nearest foot if your map is only

accurate to the nearest mile. You will end up looking for information that does not

exist.

Map Resolution

Map resolution refers to how accurately the location and shape of map features can be

depicted for a given map scale. Scale affects resolution. In a larger-scale map, the

resolution of features more closely matches real-world features because the extent of

reduction from ground to map is less. As map scale decrease, the map resolution

diminishes because features must be smoothed and simplified, or not shown at all.

Map Accuracy

Many factors besides resolution, influence how accurately features can be depicted,

including the quality of source data, the map scale, your drafting skill and the width of

lines drawn on the ground. A fine drafting pen will draw line's 1/100 of an inch wide.

Such a line represents a corridor on the ground, which is almost 53 feet wide.

In addition to this, human drafting errors will occur and can be compounded by the

quality of your source maps and materials. A map accurate for one purpose is often

inaccurate for others since accuracy is determined by the needs of the project as much

as it is by the map itself.

Some measurements of a map's accuracy are discussed below.

Absolute accuracy of a map refers to the relationship between a geographic

position on a map (a street corner, for instance) and its real-world position measured

on the surface of the earth. Absolute accuracy is primarily important for complex data

requirements such as those for surveying and engineering-based applications.


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Relative accuracy refers to the displacement between two points on a map

(both distance and angle), compared to the displacement of those same points in the

real world. Relative accuracy is often more important and easier to obtain than

absolute accuracy because users rarely need to know absolute positions. More often,

they need to find a position relative to some known landmark, which is what relative

accuracy provides. Users with simple data requirements generally need only relative

accuracy.

Attribute accuracy refers to the precision of the attribute database linked to the

map's features. For example, if the map shows road classifications, are they correct? If

it shows street addresses, how accurate are they? Attribute accuracy is most important

to users with complex data requirements.

A map's Currency refers to how up-to-date it is. Currency is usually expressed

in terms of a revision date, but this information is not always easy to find.

A map is Complete if it includes all the features a user would expect it to

contain. For example, does a street map contain all the streets? Completeness and

currency usually are related because a map becomes less complete as it gets older.

The most important issue to remember about map accuracy is that the more accurate

the map, the more it costs in time and money to develop. For example, digital maps

with coordinate accuracy of about 100 feet can be purchased inexpensively. If 1-foot

accuracy is required, a custom survey is often the only way to get it, which drives up

data-acquisition costs by many orders of magnitude and can significantly delay

project implementation - by months or even years.

Therefore, too much accuracy can be as detrimental to the success of a GIS project as

too little. Rather than focusing on the project's benefits, a sponsoring organizationmay focus on the costs that result from a level of accuracy not justified for the project.

Project support inevitably erodes when its original objectives are forgotten in a flurry

of cost analyses.

A far better strategy is to start the project with whatever data is readily available and

sufficient to support initial objectives. Once the GIS is up and running, producing

useful results, project scope can be expanded. The quality of its data can be improved

as required.


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Even though no maps are entirely accurate, they are still useful for decision-making

and analysis. How ever, it is important to consider map accuracy to ensure that your

data is not used inappropriately.

Any number of factors can cause error. Note these sources can have at cumulative

effect.

E = f(f) + f(1) + f(e) + f(d) + f(a) + f(m) + f(rms) + f(mp) + u

Where,

f = flattening the round Earth onto a two - dimensional surface (transformation from

spherical to planar geometry)

I = accurately measuring location on Earth (correct project and datum information)

c = cartographic interpretation (correct interpretation of features)

d = drafting error (accuracy in tracing of features and width of drafting pen)

a = analog to digital conversion (digitizing board calibration)

m = media stability (warping and stretching, folding. Wrinkling of map)

p = digitizing processor error (accuracy of cursor placement)

rms = Root Mean Square (registration accuracy of ties)

mp = machine precision (coordinate rounding by computer in storing and

transforming)

u = additional unexplained source error

Map Extent

The aerial extent of map is the area on the Earth's surface represented on the map. It is

the limit of the area covered, usually defined by rectangle just large enough to include

all mapped features. The size of the study area depends on the map scale. The smaller

the scale the larger the area covered.

Database Extent

A critical first step in building a geographic database is defining its extent. The aerial

extent of a database is the limit of the area of interest for your GIS project. This


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usually includes the areas directly affected by your organization's responsibility (such

as assigned administrative units) as well as surrounding areas that either influence or

are influenced by relevant activities in the administrative area.

Data Automation

Map features are logically organized into a set of layers or themes of information. A

base map can be organized into layers such as streams, soils, wells or boundaries.

Map data, regardless of how a spatial database will be applied, is collected, automated

and updated as series of adjacent map sheets or aerial photograph. Here each sheet is

mounted on the digitizer and digitized, one sheet at a time. In order to be able to

combine these smaller sheets into larger units or study areas, the co-ordinates of

coverage must be transformed into a single common co-ordinate system. Once in a

common co-ordinate system, attributes are associated with features. Then as needed

map sheets for layer are edge matched and joined into a single coverage for your

study area.

Types of Information in a Digital Map

Any digital map is capable of storing much more information than a paper map of the

same area, but it's generally not clear at first glance just what sort of information the

map includes. For example, more information is usually available in a digital map

than what you see on-screen. And evaluating a given data set simply by looking at the

screen can be difficult: What part of the image is contained in the data and what part

is created by the GIS program's interpretation of the data? You must understand the

types of data in your map so you can use it appropriately.

Three general types of information can be included in digital maps:

Geographic information, which provides the position and shapes of specific

geographic features.

Attribute information, which provides additional non-graphic information

about each feature.

Display information, which describes how the features will appear on the

screen.


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Some digital maps do not contain all three types of information. For example,

raster maps usually do not include attribute information, and many vector data sources

do not include display information.

Geographic Information

The geographic information in a digital map provides the position and shape of each

map feature. For example, a road map's geographic information is the location of each

road on the map.

In a vector map, a feature's position is normally expressed as sets of X, Y pairs or X,

Y, Z triples, using the coordinate system defined for the map (see the discussion of

coordinate systems, below). Most vector geographic information systems support

three fundamental geometric objects:

Point: A single pair of coordinates.

Line: Two or more points in a specific sequence.

Polygon: An area enclosed by a line.

Some systems also support more complex entities, such as regions, circles, ellipses,

arcs, and curves.

Attribute Information

Attribute data describes specific map features but is not inherently graphic. For

example, an attribute associated with a road might be its name or the date it was last

paved. Attributes are often stored in database files kept separately from the graphic

portion of the map. Attributes pertain only to vector maps; they are seldom associated

with raster images.

GIS software packages maintain internal links tying each graphical map entity to its

attribute information. The nature of these links varies widely across systems. In some,

the link is implicit, and the user has no control over it. Other systems have explicit

links that the user can modify. Links in these systems take the form of database keys.

Each map feature has a key value stored with it; the key identifies the specific


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database record that contains the feature's attribute information.

Display Information

The display information in a digital-map data set describes how the map is to be

displayed or plotted. Common display information includes feature colours, line

widths and line types (solid, dashed, dotted, single, or double); how the names of

roads and other features are shown on the map; and whether or not lakes, parks, or

other area features are colour coded.

However, many users do not consider the quality of display information when they

evaluate a data set. Yet map display strongly affects the information you and your

audience can obtain from the map - no matter how simple or complex the project. A

technically flawless, but unattractive or hard-to-read map will not achieve the goal of

conveying information easily to the user.

Cartographic Appeal

Clearly, how a map looks - especially if it is being used in a presentation - determines

its effectiveness. Appropriate color choices, linetypes, and so on add the professional

look you want and make the map easier to interpret. Since display information often is

not included in the source data set or is filtered out by conversion software, you may

need to add it yourself or purchase the map from a vendor who does it for you. Map

display information should convey the meaning of its underlying attribute data.

Various enhancements will increase a map's usefulness and cartographic appeal.

Feature Colors and Linetypes. Colors and line representations should bechosen to make the map's meaning clear. For example, using double-line roads can be

quite helpful. Many GIS data sets only include road centerline information. Actual

road width is not given. So maps with centerlines only can look like spider webs,

which is visually unappealing. Some software and conversion systems can draw roads

as double lines, with distance between lines varying according to road type.

Centerlines can be included, if necessary. Double-line maps are appropriate for

detailed studies of small areas, such as subdivisions, or maps where right-of-way

information is important.


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Naming Roads. Naming, or labeling, roads are important for proper map

interpretation. This information should be legible, positioned in the center of the road

or offset from the center, and drawn at intervals suited to the scale of the final map or

its purpose.

Landmark Symbols. A good set of symbols should be used to indicate

landmarks, such as hospitals, schools, churches, and cemeteries. The symbols should

be sized appropriately in relation to map scale.

Polygon Fills. Polygon features, such as lakes or parks, should be filled with

an appropriate color or hatch pattern.

Zoom Layer Control. If the GIS software platform permits, map layers should

be set up so that detailed, high-density information only appears when the user zooms

in for a close-up of part of the map. For example, when a large area is displayed, only

the major roads should appear; for a smaller area, both major and minor roads should

appear.

Layering

Most GIS software has a system of layers, which can be used to divide a large map

into manageable pieces. For example, all roads could be on one layer and all

hydrographic features on another. Major layers can be further classified into sub-

layers, such as different types of roads - highways, city streets, and so on. Layer

names are particularly important in CAD-based mapping and GIS programs, which

have excellent tools for handling them.

Some digital maps are layered according to the numeric feature-classification codes

found in their source data sets. For example, a major road might be on the 170-201

layer. However, this type of system is not very useful. A well-thought-out layeringscheme can make any data set much easier to use because it allows the user to control

the features with which you want to work. A good layering standard has layer names

that are mnemonic (suggest their meanings) and hierarchical (have a structured

classification scheme that makes it easy to choose general or specific classes).

For example, a map could have its roads on a layer called RD, its railroads on a layer

called RR, its road bridges on a layer called RD-BRIDGE, and its railroad bridges on

a layer called RR-BRIDGE. This scheme is mnemonic because it is easy to tell a


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layer's contents from its name, and it's hierarchical because the user can easily select

all the roads, railroads, bridges, road bridges, or railroad bridges.

Maps and Map Analysis

Automated Mapping

Computer Aided Mapping has its limitations. Goal of GIS is not only to prepare a

good map but also perform map analysis. Maps are the main source of data for GIS.

GIS, though an accurate mapping tool, requires error management.

MAP is a representation on a medium of a selected material or abstract material in

relation to the surface of the earth (defined by Cartographic association). Maps

originated from mathematics. The term Map is often used in mathematics to convey

the motion of transferring the information from one form to another just as

Cartographers transfer information from the surface of the earth to a sheet of paper.

Map is used in a loose fashion to refer to any manual display of information

particularly if it is abstract, generalised or schematic.

Process involved in the production of Maps:

Selection of few features of the real world.

Classification of selected features in to groups eg. Railway in to different

lines. Classification depends upon the purpose.

Simplification of jaggered lines like the coast lines.

Exaggeration of features.

Symbolisation to represent different classes of features.

Drawing Digitization of Maps.

Maps can be broadly classified in to two groups:

1. Topographical maps

2.

Thematic maps

Topographical Maps

It is a reference map showing the outline of selected man-made and natural features of


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the earth. It often acts as a frame for other features Topography refers to the shape of

surface represented by contours or shading. It also shows lands, railway and other

prominent features.

Thematic maps

Thematic maps are an important source of GIS information. These are tools to

communicate geographical concepts such as Density of population, Climate,

movement of goods and people, land use etc. It has many classifications.

Geographical Data Sets

Geographic Data Types

Although the two terms, data and information, are often used indiscriminately, they

both have a specific meaning. Data can be described as different observations, which

are collected and stored. Information is that data, which is useful in answering queries

or solving a problem. Digitizing a large number of maps provides a large amount of


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data after hours of painstaking works, but the data can only render useful information

if it is used in analysis.

Spatial and Non-spatial data

Geographic data are organised in a geographic database. This database can be

considered as a collection of spatially referenced data that acts as a model of reality.

There are two important components of this geographic database: its geographic

position and its attributes or properties. In other words, spatial data (where is it?) and

attribute data (what is it?)

Attribute Data

The attributes refer to the properties of spatial entities. They are often referred to as

non-spatial data since they do not in themselves represent location information.

District Name Area Population

Noida 395 sq. Km. 6,75,341

Ghaziabad 385 sq. Km. 2,57,086

Mirzapur 119 sq. Km. 1,72,952

Spatial data

Geographic position refers to the fact that each feature has a location that must be

specified in a unique way. To specify the position in an absolute way a coordinate

system is used. For small areas, the simplest coordinate system is the regular square

grid. For larger areas, certain approved cartographic projections are commonly used.

Internationally there are many different coordinate systems in use.

Geographic object can be shown by FOUR type of representation viz., points, lines,

areas, and continuous surfaces.

Point Data

Points are the simplest type of spatial data. They are-zero dimensional objects withonly a position in space but no length.


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Line Data

Lines (also termed segments or arcs) are one-dimensional spatial objects. Besides

having a position in space, they also have a length.

Area Data

Areas (also termed polygons) are two-dimensional spatial objects with not only a

position in space and a length but also a width (in other words they have an area).

Continuous Surface

Continuous surfaces are three-dimensional spatial objects with not only a position in

space, a length and a width, but also a depth or height (in other words they have a

volume). These spatial objects have not been discussed further because most GIS do

not include real volumetric spatial data.

Geographic Data -- Linkages and Matching

Linkages

A GIS typically links different sets. Suppose you want to know the mortality rate to

cancer among children under 10 years of age in each country. If you have one file that

contains the number of children in this age group, and another that contains the

mortality rate from cancer, you must first combine or link the two data files. Once this

is done, you can divide one figure by the other to obtain the desired answer.


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Exact Matching

Exact matching occurs when you have information in one computer file about many

geographic features (e.g., towns) and additional information in another file about the

same set of features. The operation to bring them together is easily achieved by using

a key common to both files -- in this case, the town name. Thus, the record in each

file with the same town name is extracted, and the two are joined and stored in

another file.

Name Populaiton

A 4038

B 7030

C 10777

D 5798

E 5606

Name Avg. housing Cost

A 30,500

B 22,000

C 100,000

D 24,000

E 24,000


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Name Population Avg. Housing Cost

A 4038 30,500

B 7030 22,000

C 10777 100,100

D 5798 24,000

E 5606 24,000

Hierarchical Matching

Some types of information, however, are collected in more detail and less frequently

than other types of information. For example, financial and unemployment data

covering a large area are collected quite frequently. On the other hand, population

data are collected in small areas but at less frequent intervals. If the smaller areas nest

(i.e., fit exactly) within the larger ones, then the way to make the data match of the

same area is to use hierarchical matching -- add the data for the small areas together

until the grouped areas match the bigger ones and then match them exactly.

The hierarchical structure illustrated in the chart shows that this city is composed of

several tracts. To obtain meaningful values for the city, the tract values must be added

together.

Tract Town Population

101 P 60,000

102 Q 45,000

103 R 35,000

104 S 36,000

105 T 57,000

106 Nakkhu 25,000

107 Kupondole 58,000


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Tract 101

Tract 102

Tract 103

Tract 104

Tract 105

Tract 107

Tract 106

Fuzzy Matching

On many occasions, the boundaries of the smaller areas do not match those of the

larger ones. This occurs often while dealing with environmental data. For example,

crop boundaries, usually defined by field edges, rarely match the boundaries between

the soil types. If you want to determine the most productive soil for a particular crop,

you need to overlay the two sets and compute crop productivity for each and every

soil type. In principle, this is like laying one map over another and noting the

combinations of soil and productivity.

A GIS can carry out all these operations because it uses geography, as a common key

between the data sets. Information is linked only if it relates to the same geographical

area.

Why is data linkage so important? Consider a situation where you have two data sets

for a given area, such as yearly income by county and average cost of housing for the

same area. Each data might be analysed and/or mapped individually. Alternatively,they may be combined. With two data sets, only one valid combination exists. Even if

your data sets may be meaningful for a single query you will still be able to answer

many more questions than if the data sets were kept separate. By bringing them

together, you add value to the database. To do this, you need GIS.


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Figure 2

Principal Functions of GIS

Data Capture

Data used in GIS often come from many types, and are stored in different ways. A

GIS provides tools and a method for the integration of different data into a format to

be compared and analysed. Data sources are mainly obtained from manual digitization

and scanning of aerial photographs, paper maps, and existing digital data sets.Remote-sensing satellite imagery and GPS are promising data input sources for GIS.

Database Management and Update

After data are collected and integrated, the GIS must provide facilities, which can

store and maintain data. Effective data management has many definitions but should

include all of the following aspects: data security, data integrity, data storage and

retrieval, and data maintenance abilities.

Geographic Analysis

Data integration and conversion are only a part of the input phase of GIS. What is

required next is the ability to interpret and to analyze the collected information

quantitatively and qualitatively. For example, satellite image can assist an agricultural

scientist to project crop yield per hectare for a particular region. For the same region,

the scientist also has the rainfall data for the past six months collected through

weather station observations. The scientists also have a map of the soils for the region


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which shows fertility and suitability for agriculture. These point data can be

interpolated and what you get is a thematic map showing isohyets or contour lines of

rainfall.

Presenting Results

One of the most exciting aspects of GIS technology is the variety of different ways in

which the information can be presented once it has been processed by GIS.

Traditional methods of tabulating and graphing data can be supplemented by maps

and three dimensional images. Visual communication is one of the most fascinating

aspects of GIS technology and is available in a diverse range of output options.

Data Capture an Introduction

The functionality of GIS relies on the quality of data available, which, in most

developing countries, is either redundant or inaccurate. Although GIS are being used

widely, effective and efficient means of data collection have yet to be systematically

established. The true value of GIS can only be realized if the proper tools to collect

spatial data and integrate them with attribute data are available.

Manual Digitization

Manual Digitizing still is the most common method for entering maps into GIS. The

map to be digitized is affixed to a digitizing table, and a pointing device (called the

digitizing cursor or mouse) is used to trace the features of the map. These features can

be boundary lines between mapping units, other linear features (rivers, roads, etc.) or

point features (sampling points, rainfall stations, etc.) The digitizing table

electronically encodes the position of the cursor with the precision of a fraction of a

millimeter. The most common digitizing table uses a fine grid of wires, embedded in

the table. The vertical wires will record the Y-coordinates, and the horizontal ones,

the X-coordinates.

The range of digitized coordinates depends upon the density of the wires (called

digitizing resolution) and the settings of the digitizing software. A digitizing table is

normally a rectangular area in the middle, separated from the outer boundary of the

table by a small rim. Outside of this so-called active area of the digitizing table, no


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coordinates are recorded. The lower left corner of the active area will have the

coordinates x = 0 and y = 0. Therefore, make sure that the (part of the) map that you

want to digitize is always fixed within the active area.

Scanning System

The second method of obtaining vector data is with the use of scanners. Scanning (or

scan digitizing) provides a quicker means of data entry than manual digitizing. In

scanning, a digital image of the map is produced by moving an electronic detector

across the map surface. The output of a scanner is a digital raster image, consisting of

a large number of individual cells ordered in rows and columns. For the Conversion to

vector format, two types of raster image can be used.

In the case of Chloropleth maps or thematic maps, such as geological maps, the

individual mapping units can be separated by the scanner according to their different

colours or grey tones. The resulting images will be in colours or grey tone images.

In the case of scanned line maps, such as topographic maps, the result is a black-

and-white image. Black lines are converted to a value of 1, and the white areas in

between lines will obtain a value of 0 in the scanned image. These images, with only

two possibilities (1 or 0) are also called binary images.

The raster image is processed by a computer to improve the image quality and is then

edited and checked by an operator. It is then converted into vector format by special

computer programmes, which are different for colour/grey tone images and binary

images.

Scanning works best with maps that are very clean, simple, relate to one feature only,

and do not contain extraneous information, such as text or graphic symbols. For

example, a contour map should only contain the contour line, without height

indication, drainage network, or infrastructure. In most cases, such maps will not be

available, and should be drawn especially for the purpose of scanning. Scanning and

conversion to vector is therefore, only beneficial in large organizations, where a large

number of complex maps are entered. In most cases, however, manual digitizing will

be the only useful method for entering spatial data in vector format.


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Figure 3

Data Conversion

While manipulating and analyzing data, the same format should be used for all data.

This Scanning System implies that, when different layers are to be used

simultaneously, they should all be in vector or all in raster format. Usually the

conversion is from vector to raster, because the biggest part of the analysis is done in

the raster domain. Vector data are transformed to raster data by overlaying a grid with

a user-defined cell size.

Sometimes the data in the raster format are converted into vector format. This is the

case especially if one wants to achieve data reduction because the data storage needed

for raster data is much larger than for vector data.

A digital data file with spatial and attribute data might already exist in some way or


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another. There might be a national database or specific databases from ministries,

projects, or companies. In some cases a conversion is necessary before these data can

be downloaded into the desired database.

The commonly used attribute databases are dBase and Oracle. Sometimes spreadsheet

programmes like Lotus, Quattro, or Excel are used, although these cannot be regarded

as real database softwares.

Remote-sensing images are digital datasets recorded by satellite operating agencies

and stored in their own image database. They usually have to be converted into the

format of the spatial (raster) database before they can be downloaded.Spatial Data

Management

Geo-Relational Data Model

All spatial data files will be geo-referenced. Geo-referencing refers to the location of a

layer or coverage in space defined by the coordinate referencing system. The geo

relational approach involves abstracting geographic information into a series of

independent layers or coverages, each representing a selected set of closely associated

geographic features (e.g., roads, land use, river, settlement, etc). Each layer has the

theme of a geographic feature and the database is organized in the thematic layers.

With this approach users can combine simple feature sets representing complex

relationships in the real world. This approach borrows heavily on the concepts of

relational DBMS, and it is typically closely integrated with such systems. This is

fundamental to database organization in GIS.

Topological Data Structure.

Topology is the spatial relationship between connecting and adjacent coverage

features (e.g., arc, nodes, polygons, and points). For instance, the topology of an arc

includes from and to nodes (beginning of the arc and ending of the arc representing

direction) and its left and right polygon. Topological relationships are built from

simple elements into complex elements: points (simplest elements), arcs (sets of

connected points), and areas (sets of connected arcs). Topological data structure, in

fact, adds intelligence to the GIS database.


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Attribute Data Management

All Data within a GIS (spatial data as well as attribute data) are stored within

databases. A database is a collection of information about things and their

relationships to each other. For example, you can have an engineering geological

database, containing information about soil and rock types, field observations and

measurements, and laboratory results. This is interesting data, but not very useful if

the laboratory data, for example, cannot be related to soil and rock types.

The objective of collecting and maintaining information in a database is to relate facts

and situations that were previously separate.

The principle characteristics of a DBMS are: -

Centralized control over the database is possible, allowing for better quality

management and operator-defined access to parts of the database;

Data can be shared effectively by different applications;

The access to the data is much easier, due to the use of a user-interface and the user-

views (especially designed formula for entering and consulting the database);

Data redundancy (storage of the same data in more than one place in the database) can

be avoided as much as possible; redundancy or unnecessary duplication of data are an

annoyance, since this makes updating the database much more difficult; one can

easily overlook changing redundant information whenever it occurs; and

The creation of new applications is much easier with DBMS.

The disadvantages relate to the higher cost of purchasing the software, the increased

complexity of management, and the higher risk, as data are centrally managed.

Relational Database -- Concepts & Model

The relational data model is conceived as a series of tables, with no hierarchy nor any

predefined relations. The relation between the various tables should be made by the


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user. This is done by identifying a common field in two tables, which is assigned as

the flexibility than in the other two data models. However, accessing the database is

slower than with the other two models. Due to its greater flexibility, the relational data

model is used by nearly all GIS systems

Choosing geographic data

The main purpose of purchasing a geographic information system (GIS)* is to

produce results for your organization. Choosing the right GIS/mapping data will help

you produce those results effectively.

The role of base-map data in your GIS,

The common characteristics of geographic data,

The commonly available data sources

Guidelines for evaluating the suitability of any data set for your project.

The world of GIS data is complex, by choosing the right data set, you can save

significant amounts of money and, even more importantly, quickly begin your GIS

project.

Data: The Core of Your Mapping / GIS Project

When most people begin a GIS project, their immediate concern is with purchasing

computer hardware and software. They enter into lengthy discussions with vendors

about the merits of various components and carefully budget for acquisitions. Yet

they often give little thought to the core of the system, the data that goes inside it.

They fail to recognize that the choice of an initial data set has a tremendous influence

on the ultimate success of their GIS project.

Data, the core of any GIS project, must be accurate - but accuracy is not enough.

Having the appropriate level of accuracy is vital. Since an increase in data accuracy

increases acquisition and maintenance costs, data that is too detailed for your needs

can hurt a project just as surely as inaccurate data can. All any GIS project needs is

data accurate enough to accomplish its objectives and no more. For example, you


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would not purchase an engineering workstation to run a simple word-processing

application. Similarly, you would not need third-order survey accuracy for a GIS-

based population study whose smallest unit of measurement is a county. Purchasing

such data would be too costly and inappropriate for the project at hand. Even more

critically, collecting overly complex data could be so time-consuming that the GIS

project might lose support within the organization.

Even so, many people argue that, since GIS data can far outlast the hardware and

software on which it runs, no expense should be spared in its creation. Perfection,

however, is relative. Projects and data requirements evolve. Rather than overinvest in

data, invest reasonably in a well-documented, well-understood data foundation that

meets today's needs and provides a path for future enhancements. This approach is a

key to successful GIS project implementation.

Are Your Data Needs Simple or Complex?

Before you start your project, take some time to consider your objectives and your

GIS data needs. Ask yourself, "Are my data needs complex or simple?"

*Italicized words can be found in the Glossary at the end of this document except for

words used for emphasis or words italicized for reasons of copyediting convention or

layout.

If you just need a map as a backdrop for other information, your data requirements are

simple. You are building a map for your specific project, and you are primarily

interested in displaying the necessary information, not in the map itself. You do not

need highly accurate measurements of distances or areas or to combine maps from

different sources. Nor do you want to edit or add to the map's basic geographic

information.

An example of simple data requirements is a map for a newspaper story that shows

the location of a fire. Good presentation is important; absolute accuracy is not.

If you have simple data needs, read this paper to get the overall picture of what GIS

data is and how it fits into your project. A project with simple data requirements can


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be started with inexpensive maps. Your primary interests will be quality graphic-

display characteristics and finding maps that are easy to use with your software. You

need not be as concerned with technical mapping issues. However, basic knowledge

of concepts such as coordinate systems, absolute accuracy, and file formats will help

you understand your choices and help you make informed decisions when it's time to

add to your system.

What issues suggest more complex GIS data needs?

Building a GIS to be used by many people over a long period of time.

Storing and maintaining database information about geographic features.

Making accurate engineering measurements from the map.

Editing or adding to the map.

Combining a variety of information from different sources.

An example of a system requiring complex data would be a GIS built to manage

infrastructure for an electric utility.

If your data requirements are complex, you ought to pay particular attention to the

sections of this paper that discuss data accuracy, coordinate systems, layering, file

formats, and the issues involved in combining data from different sources.

Also keep in mind that projects evolve, and simple data needs expand into complex

ones as your project moves beyond its original objectives. If you understand the

basics of your data set, you will make better decisions as your project grows.

Basics of Digital Mapping

Vector vs. Raster Maps

The most fundamental concept to grasp about any type of graphic data is making the

distinction between vector data and raster data. These two data types are as different

as night and day, yet they can look the same. For example, a question that commonly

comes up is "How can I convert my TIFF files into DXF files?" The answer is "With

difficulty," because TIFF is a raster data format and DXF (data interchange file) is


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a vector format. And converting from raster to vector is not simple. Raster maps are

best suited to some applications while vector maps are suited to others.

Figure 4

Raster data represents a graphic object as a pattern of dots, whereas vector data

represents the object as a set of lines drawn between specific points. Consider a line

drawn diagonally on a piece of paper. A raster file would represent this image by

subdividing the paper into a matrix of small rectangles-similar to a sheet of graph

paper-called cells (figure 1). Each cell is assigned a position in the data file and given

a value based on the color at that position. White cells could be given the value 0;black cells, the value 1; grays would fall in-between. This data representation allows

the user to easily reconstruct or visualize the original image.

Figure 5

A vector representation of the same diagonal line would record the position of the line

by simply recording the coordinates of its starting and ending points. Each point

would be expressed as two or three numbers (depending on whether the representation

was 2D or 3D, often referred to as X,Y or X,Y,Z coordinates (figure 2). The first


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number, X, is the distance between the point and the left side of the paper; Y, the

distance between the point and the bottom of the paper; Z, the point's elevation above

or below the paper. The vector is formed by joining the measured points.

Some basic properties of raster and vector data are outlined below.

Each entity in a vector file appears as an individual data object. It is easy to

record information about an object or to compute characteristics such as its exact

length or surface area. It is much harder to derive this kind of information from a

raster file because raster files contain little (and sometimes no) geometric information.

Some applications can be handled much more easily with raster techniques

than with vector techniques. Raster works best for surface modeling and for

applications where individual features are not important. For example, a raster surface

model can be very useful for performing cut-and-fill analyses for road-building

applications, but it doesn't tell you much about the characteristics of the road itself.

Terrain elevations can be recorded in a raster format and used to construct digital

elevation models (DEMs) (figure 3). Some land-use information comes in raster

format.

Figure 6

Raster files are often larger than vector files. The raster representation of the

line in the example above required a data value for each cell on the page, whereas the

vector representation only required the positions of two points.


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The size of the cells in a raster file is an important factor. Smaller cells improve image

quality because they increase detail. As cell size increases, image definition decreases

or blurs. In the example, the position of the line's edge is defined most clearly if the

cells are very small. However, there is a trade-off: Dividing the cell size in half

increases file size by a factor of four.

Cell size in a raster file is referred to as resolution. For a given resolution value, the

raster cost does not increase with image complexity. That is, any scanner can quickly

make a raster file. It takes no more effort to scan a map of a dense urban area than to

scan a sparse rural one. On the other hand, a vector file requires careful measuring

and recording of each point, so an urban map will be much more time-consuming to

draw than a rural map. The process of making vector maps is not easily automated,

and cost increases with map complexity.

Because raster data is often more repetitive and predictable, it can be compressed

more easily than vector data. Many raster formats, such as TIFF, have compression

options that drastically reduce image sizes, depending upon image complexity and

variability.

Raster files are most often used:

For digital representations of aerial photographs, satellite images, scanned

paper maps, and other applications with very detailed images.

When costs need to be kept down.

When the map does not require analysis of individual map features.

When "backdrop" maps are required.

In contrast, vector maps are appropriate for:

Highly precise applications.

When file sizes are important.

When individual map features require analysis.

When descriptive information must be stored.


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Raster and vector maps can also be combined visually. For example, a vector street

map could be overlaid on a raster aerial photograph. The vector map would provide

discrete information about individual street segments, the raster image, a backdrop of

the surrounding environment.

Digital Map Formats- How Data Is Stored

The term file format refers to the logical structure used to store information in a GIS

file. File formats are important in part because not every GIS software package

supports all formats. If you want to use a data set, but it isn't available in a format that

your GIS supports, you will have to find a way to transform it, find another data set,

or find another GIS.

Almost every GIS has its own internal file format. These formats are designed for

optimal use inside the software and are often proprietary. They are not designed for

use outside their native systems. Most systems also support transfer file formats.

Transfer formats are designed to bring data in and out of the GIS software, so they are

usually standardized and well documented.

If your data needs are simple, your main concern will be with the internal format that

your GIS software supports. If you have complex data needs, you will want to learn

about a wider range of transfer formats, especially if you want to mix data from

different sources. Transfer formats will be required to import some data sets into your

software.

Vector Formats

Many GIS applications are based on vector technology, so vector formats are the most

common. They are also the most complex because there are many ways to store

coordinates, attributes, attribute linkages, database structures, and display

information. Some of the most common formats are briefly described below


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Common Vector File Formats

Format NameSoftware

Platform

Internal or

TransferDeveloper Comments

Arc Export ARC/INFO* Transfer

Environmental

Systems Research

Institute, Inc. (ESRI)

Transfers data

across ARC/INFO*

platforms.

ARC/INFO* Coverages ARC/INFO* Internal ESRI

AutoCAD Drawing Files

(DWG)AutoCAD* Internal Autodesk

Autodesk Data

Interchange File

(DXF)

Many Transfer Autodesk

Widely used

graphics transfer

standard.

Digital Line graphs

(DLG)Many Transfer

United States

Geological Survey

(USGS)

Used to publish

USGS digital maps.

Hewlett-Packard

Graphic Language

(HPGL)

Many Internal Hewlett-PackardUsed to control HP

plotters.

MapInfo Data TransferFiles (MIF/MID)

MapInfo* Transfer MapInfo Corp.

MapInfo Map Files MapInfo* Internal MapInfo Corp.

MicroStation Design

Files (DGN)MicroStation* Internal Bentley Systems, Inc.

Spatial Data Transfer

System (SDTS)

Many (in the

future)Transfer US Government

New US standard

for vector and raster

geographic data.

Topologically Integrated

Geographic Encoding

and Referencing

(TIGER)

Many Transfer US Census Bureau

Used to publish US

Census Bureau

maps.

Vector Product Format

(VPF)

Military

mapping

systems

BothUS Defense Mapping

Agency

Used to publish

Digital Chart of the

World.


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Raster Formats

Raster files generally are used to store image information, such as scanned paper

maps or aerial photographs. They are also used for data captured by satellite and other

airborne imaging systems. Images from these systems are often referred to as remote-

sensing data. Unlike other raster files, which express resolution in terms of cell size

and dots per inch (dpi), resolution in remotely sensed images is expressed in meters,

which indicates the size of the ground area covered by each cell.

Some common raster formats are described below

Format NameSoftware

PlatformInternal or

TransferDeveloper Comments

Arc DigitizedRaster Graphics(ADRG)

Militarymappingsystems

Both US DefenseMapping Agency

Band Interleavedby Line (BIL)

Man BothCommon remote-sensing standard.

Band Interleavedby Pixel (BIP)

Many BothCommon remote-sensing standard.

Band Sequential(BSQ)

Many BothCommon remote-sensing standard.

Digital ElevationModel for

(DEM)

Many TransferUnited StatesGeological Survey

(USGS)

USGS standard format digital

terrain models.

PC PaintbrushExchange (PCX)

PC Paintbrush Both Zsoft Widely used raster format.

Spatial DataTransfer

Standard (SDTS)

Many (in thefuture)

TransferUS FederalGovernment

New US standard for both rasterand vector geographic data;raster version still underdevelopment.

Tagged ImageFile Format(TIFF)

PageMaker Both Aldus Widely used raster format.


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An Example of Raster and Vector Integration

Figure 7: An Example of Raster and Vector Integration

Vectors & Raster Data Models - Merits & Demerits.

RASTER MODEL VECTOR MODEL

Advantages

Simple data structureEasy and efficient overlaying

Compatible with RS imageryHigh spatial variability is efficiently

representedSimple for own programmingSame grid cells for several attributes

Disadvantages

Inefficient use of computer storageErrors in perimeter, and shapeDifficult network analysisInefficient projection transformationsLoss of information when using large

cells Less accurate (although interactive) maps

Advantages

Compact data structureEfficient for network analysis

Efficient projection transformationAccurate map output.

Disadvantages

Complex data structureDifficult overlay operations

High spatial variability is inefficientlyrepresented

Not compatible with RS imagery


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Hybrid System

It is an integration of the best of Vector and Raster Models. The GIS technology is

fast moving towards Hybrid model GIS.

The Integration of Vector and Raster System Hybird System

Figure 8: The Integration of Vector and Raster System Hybird System


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Analysis of Geographic Data

ANALYSIS - What? & Why?

The heart of GIS is the analytical capabilities of the system. What distinguish the GIS

system from other information system are its spatial analysis functions. Although the

data input is, in general, the most time consuming part, it is for data analysis that GIS

is used. The analysis functions use the spatial and non-spatial attributes in the

database to answer questions about the real world. Geographic analysis facilitates the

study of real-world processes by developing and applying models. Such models

illuminate the underlying trends in geographic data and thus make new information

available. Results of geographic analysis can be communicated with the help of maps,

or both.

The organization of database into map layers is not simply for reasons of

organizational clarity, rather it is to provide rapid access to data elements required for

geographic analysis. The objective of geographic analysis is to transform data into

useful information to satisfy the requirements or objectives of decision-makers at all

levels in terms of detail. An important use of the analysis is the possibility of

predicting events in the another location or at another point in time.

ANALYSIS - How?

Before commencing geographic analysis, one needs to assess the problem and

establish an objective. The analysis requires step-by-step procedures to arrive at the

conclusions.

The range of geographical analysis procedures can be subdivided into the following

categories.

Database Query.

Overlay.

Proximity analysis.

Network analysis.

Digital Terrain Model.


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Statistical and Tabular Analysis.

Spatial Analysis

It helps us to:

Identify trends on the data.

Create new relationships from the data.

View complex relationships between data sets.

Make better decisions.

Geographic Analysis

Analysis of problems with some Geographic Aspects.

Alternatives are geographic locations or areas.

Decisions would affect locations or areas.

Geographic relationships are important in decision-making or modelling.

Some examples of its application:

Nearest Neighbour.

Network distances.

Planar distances.

Spatial Analysis - An Application


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Image 1

Where should we build a road from a point A to point B?

How do we minimise the impacts of building this road?

Relationship of Modelling to Analysis

Decision Models search through potential alternatives to arrive at a

recommendation.

Decision support models process raw data into forms that are directly relevant

to decision making.

Data characterisation models are used to develop a better understanding of a

system to help characterise a problem or potential solutions.

Difficulties of Geographic Analysis

Plenty of data.

Spatial relationships are important but difficult to measure. Inherent uncertainty due to scale.


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any data sources.

Difficult to make data sources compatible.

Difficult mathematics.

Quantity vs. Quality Questions.

Multiple objectives.

GIS can address some (but not all) of these difficulties.

Network Analysis

Network models are based on interconnecting logical components, of which the most

important are:

1. "Nodes" define start, end, and intersections

2. "Chains" are line features joining nodes

3. "Links" join together points making up a chain.

This network can be analyzed using GIS.A simple and most apparent network

analysis applications are:

Street network analysis,

Traffic flow modelling,

Telephone cable networking,

Pipelines etc.

The other obvious applications would be service centre locations based on travel

distance.

Basic forms of network analysis simply extract information from a network. More

complex analysis, process information in the network model to derive new

information. One example of this is the classic shortest-path between two points. The

vector mode is more suited to network analysis than the raster model.

A Road Network


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Image

Tabular Statistical Analysis

If in the above road network we have categorised the streets then in such a case the

statistical analysis answers questions like

What unique categories do I have for streets?

How many features do I have for each unique category?

Summarize by using any attribute?

Database Query

The selective display and retrieval of information from a database are among the

fundamental requirements of GIS. The ability to selectively retrieve information from

GIS is an important facility. Database query simply asks to see already stored

information. Basically there are two types of query most general GIS allow: viz.,

Query by attribute,

Query by geometry.

Map features can be retrieved on the basis of attributes, For example, show all the

urban areas having the population density greater than 1,000 per square kilometer,

Many GIS include a sophisticated function of RDBMS known as Standard Query

Language (SQL), to search a GIS database. The attribute database, in general, is


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stored in a table (relational database mode.) with a unique code linked to the

geometric data. This database can be searched with specific characteristics. However,

more complex queries can be made with the help of SQL.

GIS can carry out a number of geometric queries. The simplest application, for

example, is to show the attributes of displayed objects by identifying them with a

graphical cursor. There are five forms of primitive geometric query: viz.,

Query by point,

Query by rectangle,

Query by circle,

Query by line,

Query by polygon,

A more complex query still is one that uses both geometric and attributes search

criteria together. Many GIS force the separation of the two different types of query.

However, some GIS, using databases to store both geometric and attribute data, allow

true hybrid spatial queries.

Overlay Operations

The hallmark of GIS is overlay operations. Using these operations, new spatial

elements are created by the overlaying of maps.

There are basically two different types of overlay operations depending upon data

structures:

Raster overlayIt is a relatively straightforward operation and often many data sets

can be combined and displayed at once.

Vector overlayThe vector overlay, however is far more difficult and complex and

involves more processing.

Logical Operators

The concept of map logic can be applied during overlay. The logical operators are

Boolean functions. There are basically four types of Boolean Operators: viz., OR,


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Conditional operators were already used in the examples given above. The all

evaluate whether a certain condition has been met.

= eq 'equal' operator

ne 'non-equal' operator

< lt 'less than' operator

gt 'greater than' operator

>= ge 'greater than or equal' operator

Many systems now can handle both vector and raster data. The vector

Documents

Overview of Gis