Understanding and the functionality of GIS

Richard Knight,Biodiversity and Conservation Biology,University of the Western Cape

Email: knight.rich[at]gmail.com

The basic data type in a GIS reflects traditional data found on a map

• Attribute Data: descriptive information about a location that can be added and edited (a table)

• Spatial data: describes the absolute and relative location of geographic features (a drawing)

GIS DATA TYPES

Attribute data

Spatial data

DIFFERENCE BETWEEN GIS AND MAPS/ATLASES

• Maps printed in large formats

• Maps and Atlases are not seamless

• Maps printed at fixed scales

• Maps cannot provide all required annotation

• Maps include a scale bars but still difficult to calculate the lengths of features and almost impossible to determine areas

Traditionally spatial data has been stored and presented in the form of a map. Three basic types of spatial data models have been developed for storing geographic data digitally. These are referred to as:

1. Vector2. Raster 3. Images

In association there are1. Databases2. Metadata

SPATIAL DATA MODELS

• Raster formats have an internal format that looks like a grid /spreadsheet and is suitable for modeling

• Vector formats are defined by nodes each with an XY location

• Images are similar to a raster but does not have the internal structure for analysis (e.g. usually are integers from 0-255)

• Database relate various attribute data using relationships

• Metadata describes how the data was obtained and to be used

OVERVIEW OF FORMATS

VECTOR FORMATS • Vector GIS use points, lines and

polygons to represent features.

• Since such features are precisely define by geographical co-ordinates, they are useful for accurate calculation of measurements and are easily attached to tabulated data for querying.

• Vector-based GIS are used extensively for management such as municipal offices would require.

**

VECTOR DIAGRAMSequential points/vertex

Vertex consists of X & Y coordinate

Polygon

Line

Point

SpatialData

RASTER FORMATS

Uses “pixels” for location and value attributes and includes satellite-images and digital aerial photos are already in

this format.

Each grid will have a value that corresponds to some feature, for example water might have a value of 6 and there fore all grids which have a value of 6 represents water. Raster-based GIS systems are not strong on the data-base functionality, but are good for spatial analysis, modeling and visualizations.

MORE ON RASTER FORMATS Geographic area divided into cells

Implies regularly spaced grids

Size of cell based on data accuracy and resolution

Too large a cell size – information lost

Too small – lots of storage space, longer to process

IMAGE DATA AND GIS• These are usually used to help orientate users or to provide the

most up-to-date “view” of land features or a more “realistic” • Images are always stored digitally as pixels (a rater based system

which) and at a particular resolution and generally are not used for manipulation.

• Images can be acquired from satellites with either fairly course resolution (example LANDSAT tm with 30 m cell resolution).

• LANDSAT images are both freely available or can be purchased) Commercial satellite images such as from IKONIS, THUNDERBIRD and SPOT have much higher resolution but are very expensive to acquire.

• It should be realized that these higher resolution image contain vastly more data since as the resolution is increased three-fold (comparing a 10 m SPOT panchromatic with a LANDSAT tm image) the data volume is increased nine-fold.

The basic data type in a GIS reflects traditional data found on a map

• Spatial data: describes the absolute and relative location of geographic features (a drawing)

• Attribute Data: descriptive information about a location that can be added and edited (a table)

GIS DATA TYPES

The basic data type in a GIS reflects traditional data found on a map

• Spatial data: describes the absolute and relative location of geographic features (a drawing)

• Attribute Data: descriptive information about a location that can be added and edited (a table)

SPATIAL ANALYSIS

RELATIONAL DATA STRUCTURES EXPLAINED

• Relational database should improve data-base management. • New Database Management Systems DBMS hides the physical

details of the data structures making it easier for users to use. • Most vector-based GIS have a structure where each layer or

coverage is stored as a separate suite of files. Typically this will include a file that contains the drawn (map) elements, a file that contains the tabulated information and as a minimum a third file which establishes the links between the two files.

• Often there are further files to provide for indexing of information, for how data is “rendered” and projection information. Map customization and projections will be discussed latter.

• The implication is a GIS rendering of ten data layers is actually a co-ordinated, simultaneous representation of at least forty files!

METADATA • Metadata is described as a dataset describing other data.

• There are various standards for preparing metadata.• Each coverage or layer should posses its own metadata. • Metadata preparation usually report on

• where the data was collected, • what datum, projection and co-ordinate units, • accuracy of the information and scale at which it was

prepared, • how the data was captured (original datums,

projections, co-ordinates, including the software used).

• who prepared the data, • a history of updates • usage/distribution rights.

GIS DATA COMPRESSION• In GIS is the large quantities of the data needs to be stored. • GIS coverages represent large files and rater files of over a

terabyte are relatively easily generated. • Since raster images are especially large, considerable research

into data compression has occurred. • Advantages of data compression are not limited to reducing hard

disk space but also have benefits in faster processing. • All Internet images are compressed to achieve these benefits with

the use of GIF, JPEG and PNG formats, but these do not allow the image to be regenerated and data losses can be significant.

• More modern compression use wavelet technology (ECW and Mr Sid formats) linked to the computer’s screen resolution.

• Consequently even if the raster/image file consists of hundreds of thousands of rows and columns of pixels you will only be able to view the 800 by 1024 pixel limits of the screen.

• A GIS a user can combine information that they wish

• Each feature of a map is stored in a GIS in a series of files that are collectively referred to as a “Layer” or “Coverage”.

• Consequently a user can concentrate on the information that is relevant to the inquiry, this is facilitated by the ability to work at any chosen scale and the flexibility of being able to add or remove labels of features.

• Further a user can change the colour and style of lines, the colour and shading properties of polygons representing areas, the colour, font, size and orientation of labels and even symbol, including making customised symbols and using small image graphics.

• Users can do simple analysis like creating a 1 km buffer around the river mouth as an area of high sensitivity to an oil spill.

GIS CUSTOMIZING

GIS MAPS ARE SEARCHABLE

• Searches can be simple such as finding all estuaries which are always open (and therefore especially sensitive to an oil spill)

• Can be compound such as to find all estuaries that are always open and have a mangrove community (mangrove community).

• Can search for all features which possess certain inherent object parameters such as all line segments of the coastline that are longer than 5 km and are characterized as coarse-grained sandy beaches

• This allows users to identify both individual features that meet specified criteria or groups of features with shared parameters or features.

• Users can also search all features that are within a certain a distance from a specified point e.g. all estuaries that are within 5 km of a certain point located on the map.

GIS MAPS ARE UPDATABLE • Since the information is stored electronically and the user can

define their requirements via the software protocols information is more quickly updateable and information collected only hours previously can be used by a networked office.

• Alternatively by simply writing the information to CD the latest information can dispatched to offices outside the local network and this is also far quicker than waiting for the information to be published and distributed as an atlas.

• More recently advance in GIS applications allow information to be updated and available to the entire users of the World Wide Web.

• Consequently information can be maintained in its most current form and optimizes decision-making.

COMPUTER REQUIREMENTS

• The computer identifies as fat and thin applications • a thin client computer using a web-application GIS can be light

on resources (memory, processor or hard disk capacity) and a modern Netbook is adequate.

• On a thin client could run light applications like Arc Explorer which allows viewing and customizing of data

• A true desktop GIS systems would Windows XP with 2 GB of memory or using Windows Vista with 4 GB of memory together with adequate disk space to store the data and some out of office back-up of data.

• The web-based GIS server’s hardware would include dual-processor xenon-based IBM Netfinity Server with 4.3 Gbytes of RAM and multiple raided hard drives (vs they specialised enterprise-level servers).

CONVERTING MAPS AND DATA INTO A GIS

• Geocoding one Table against an already existing GIS table and feature list

• Using a Digitizing Board

• Scan a pre-existing map and then geo-reference it within the GIS

• Using a GPS to field map features

DATA SOURCES • Obtained from government and commercial vendors.

• Data supplied via internet services, or via online viewing and downloading of files, e.g. http://bgis.sanbi.org.

• Meta databases – which can be maps which provide information on suppliers of spatial information.

• Can add collected GPS data

• Can mix locally stored data with that on the Web Map

• Can now digitize on-screen using a web map service

SOURCES OF DIGITAL IMAGES• North America - USGS provide togographical maps,

satellite imagery, correct orthophotos, scanned copies of paper maps, etc.

• Other sources include NOAA (weather and oceanographic information), Census Bureau, National Imagery and Mapping Agency and the Environmental Protection Agency

• South Africa – Chief Directorate Surveys and Mapping provide the above products and charge only the cost of CD cutting.

• The website http://daac.gsfc.nasa.gov/ has a large amount of available satellite data. Currently most LANDSAT data is also available for electronic downloading

ADDING NEW DATA • The USGS develops from aerial photography, digital, geo-

corrected orthophotographs of pan imagery at 1 metre resolution for the entire country every 5 years.

• In South Africa many parts have orthophotographs prepared. The most accurate digital elevation model which records height data was for a resolution of 200 metre raster grid cells from the Surveyor General for South Africa.

• More Accurate Digital Terrain Information can be acquired using the LIDAR (Light Detection and Ranging) with centimeter resolution for small areas are being prepared.

• Virtually all new maps being prepared nowadays are done through a GIS and hand drawing is very much a thing of the past.

OV

ER

LAY

S

MAP SCALE

Map scale is the relationship between the dimensions of a map and the dimensions of the Earth. It is usually expressed as a ratio between a distance on the map and a distance on the Earth, like 1:50 000.

SMALL/LARGE SCALE MAPSA large scale map is one in which a given part of the Earth is represented in more detail on the map. Large scale maps generally show more detail than small scale maps because at a large scale there is more space on the map in which to show features. Large scale maps are typically used to show site plans, local areas, neighbourhoods, towns and cities. 1:2,500 is an example of a large scale.

A small scale map is one in which a given part of the Earth is represented by less detail on the map. Small scale maps cover large parts of the Earth. Maps with regional, national, and international extents typically have small scales, such as 1:1,000,000.

Change in map extent (zoom in or out), the map scale will change as well and the scale bar will update

SPATIAL POSITIONING• Longitude run from poles & distance E to W from Prime

Meridian. 180°E/W • Latitude run East to West – tell you how far up (north) or

down (south) you can go. Measured at equator. 90 °N/S • 1 degree = 60 minutes. 1 minute = 60 seconds

COORDINATES

• Cape Town coordinates

• Y coordinate (degrees latitude) 34°17’09’’S

• X coordinate (degrees longitude) 18°56’32’’E

 

WHAT IS GPS?• Global Positioning System (GPS) • Originally dvlpd by US military – built in error (accuracy) –

selective availability (discontinued) • Worldwide radio-navigation system - 27 satellites (24 in use & 3

spares), ground stations & receivers

Orbit of GPS satellites (transmit info location) - Earth’s equator by 55º. 4 satellites visible 15° above horizon

Ground stations – tracks (exact position) & maintains satellites in space (operation health)

Receiver – collects data from satellitesDoesn’t send info

HOW A GPS WORKS?

• Satellite – transmits unique signal • GPS receiver measures distance from @

least four satellites• Distance = signal travels at speed of light,

length of time signal takes to arrive at your location

• Four satellites measure distance – four spheres drawn. Intersection = location.

• Position of GPS receiver pinpointed down to 4 m

GPS TRIANGULATION• 16 000 km from satellite A - earth imaginary sphere

with 16 000 km radius. • 18 000 km from satellite B, overlap 1st sphere with

another, larger sphere. - intersect• Distance of 3rd, 3rd sphere, which intersects with this

circle at two points. • Receivers generally

look to +4 satellites, improve accuracy - precise altitude info

MAPPING WITH GPS DATA• Note the format of

spreadsheet• Decimal degrees• South negative,

East positive• Y coordinate =

latitude = N/ Hemisphere

• X coordinate = longitude = E/W

Comma separated value fileCan be exported from Excel

MAPPING WITH POINTS

Comma separated value file. Can be exported from Excel

MAPPING WITH POINTS

Work to five decimal points to get good geographical accuracy

MAP PROJECTIONS • Representation of curved surface on a two dimensional

surface• Mathematical functions applied to transform sphere into

plane• Result distortion (length, shape, area & angle/direction)

DO

N’T

US

E

GO

OG

LE E

AR

TH

PROJECTIONS: AREA

• Many map projections are developed to be an equal area representation of the real world. They distort other spatial information in some way.

• Shape, direction or scale are distorted in order to achieve the equal area criteria.

• Albers and Azimuthal Lambert and are equal area conic projections.

PROJECTIONS: SHAPE

• Projections which represent the shape of features are referred to as conformal.

• Conformal projections usually maintain the accuracy of relative directions.

• Most large-scale maps are prepared using conformal projections.

• Lambert conformal conic is a good example of this projection.

PROJECTIONS: DISTANCE

• Projections which correctly represent the lengths between two points are referred to as equidistant.

• Equidistant projections are useful for calculating and summarizing lengths and perimeter measurements of features.

PROJECTIONS: DIRECTION

• Projections which correctly depict directions (azimuths) between points on the map and its centre rare referred to as azimuthal.

• These projections will distort one of the other maps parameters, but will represent all routes from the centre to other points as straight lines.

• Mercators projection work on these assumptions are derived from estimates based on cylindrical estimates.

Essential features:• Area relationships are maintained. Linear or

distance distortion often occurs. • Shape is often skewed. • Intersections of meridians and parallels are

not at right angles. • The map is most distorted at the edges. • Used when you want to see the distribution

of a variable by land area (e.g. population density).

EQUAL AREA PROJECTIONS

CONFORMAL PROJECTIONSEssential features:• Angles are preserved around points and the shapes

of small areas are maintained.• Meridians intersect parallels at right angles. • Scale is the same in all directions about a point (but

the scale may change from point to point). • Shapes that cover large areas are distorted. • Area is distorted. • Used for navigation (want to maintain a set angle)

and mapping phenomena with radial patterns (e.g. radio broadcast areas, wind directions, etc).

EQUIDISTANT PROJECTIONS

Essential features:

• Great circle distances are preserved. • Distance can be held true from one point to all

other points or from a few points to others but not from all points to all other points.

• Scale is uniform along the lines where distances are held true.

• Used in airplane navigation.

AZIMUTHAL PROJECTIONS

Essential features:

• True directions are preserved from one central point to all other points.

• Directions (or azimuths) from points other than the central point are not true.

• Azimuthality can occur along with equivalency, conformality, and equidistance.

• They are used in navigation and large-scale map series (USGS).

PROJECTION CHARACTERISTICS

• The simplest map projections use geometric shapes, which can be flattened without stretching their surfaces. These shapes include cylinders, cones and planes. These shapes can either be a tangent or a secant to the sphere of the Earth.

• In a tangent projection, the shape just touches the surface at either a line or a point.

• In the secant case, the shape intersects as two circles (or as one circle in the case of a plane). The place of intersection is the area of least distortion in portraying features on the earth’s surface.

Cylindrical projections are derived from projecting a spherical surface onto a cylinder. For example if you took you’re orange and wrapped an A4 sheet of paper around it. The paper can be arranged around the orange in a variety of arrangements

CYLINDRICAL PROJECTIONS

When the cylinder upon which the sphere is projected is at right angles to the poles, the cylinder and resulting projection are transverse.

TANGENT CYCLINDRICAL PROJECTION

When the cylinder is at some other, non-orthogonal, angle with respect to the poles, the cylinder and resulting projection is oblique.

ORTHOGONAL CYLINDRICAL PROJECTION

The Mercator projection is one of the best known and has straight meridians and parallels that intersect at right angles. Scale is true at the equator or at two standard parallels equidistant from the equator. This projection seriously distorts distances and areas.

MERCATOR PROJECTION

• The Universal Transverse Mercator (UTM) is probably the best known projection system for displaying large surfaces of the earth since it provides high levels of precision.

• To minimize the distortion the cylinder is wrapped around the earth transversely and is place at 60 of rotation East and West of 1800 meridian for each hemisphere.

• Consequently 60 zones north and 60 zones south are generated and are numbered eastward from the 1800 meridian. Cape Town lies in the 34th Zone and is referred to as UTM 34S. The UTM system is only applied from 840 North to 800 South Latitude.

UNIVERSAL TRANSVERSE MERCATOR

Conic projections which result from projecting a spherical surface onto a cone. When the cone is tangent to the sphere contact is along a small circle such as a latitude. You can view this by twisting your A4 sheet into a cone and placing over the orange.

CONIC PROJECTION

These are where a flat sheet is placed in contact with a sphere, and points are projected from the sphere to the sheet. You can do this by taking your A4 sheet and pressing it against the orange.

AZIMUTHAL OR PLANER PROJECTION

Plane: (Cartesian) - not a projection but truth to earth surface - data may be stored in this form, but it is not good for accurate measurements of distance e.g. metres.

NON PROJECTIONS

1. While we often refer to the earth as a sphere, it is more correctly referred to as a geoid (defined as a hypothetical surface of the earth that corresponds to mean sea level).

2. The earth is not a sphere since it is flattened at both poles and bulges at the equator. In addition there are significant bulges and depressions on the surface.

3. The are hundreds of different datums which have been used to estimate the size (areas and distances) of features on the earth.

DATUMS

To best describe this geoid mathematically, we use reference ellipsoids to approximate the size and shape of the earth.

GEOIDS AND ELLIPSOIDS

UNITS OF MEASUREMENT• Includes metres, kilometers, feet, miles etc or

expressed as degrees.• Considerations of projection, datum and units of

measurement are important since older GIS software (e.g ESRI ArcView 3 series) does not allow multiple projections to be displayed simultaneously

• Many problems for professional GIS users arises from confusion over projections and datums.

• Consequently many users store their data in Latitudes and Longitudes that are based on the WGS 84 datum.

ARCVIEW SHAPE FORMATA GIS such as Arcview is extremely widely used

Format of DataSHP – main file (info on feature geometry & geo location)DBF – attribute info in database format (open in excel)SHX – index file. connects shapefile and attribute tableAVL – legend filePRJ – projection file (stores spatial ref info re: coordinate system e.g. UTM. Not all shapefiles have PRJ)APR – project fileMetadata – data about data but use various formats and not required.

ARCVIEW PRODUCTS1. Project is a file that contains exactly the various

customization that a user has done to the layer during an ArcView session.

2. This would include which layers are switched on and how they are rendered (colour, pattern and labeling).

3. Once a user opens the Project, ArcView repeats the procedures exactly as the user previously carried out.

4. An ArcView project does not contain data. It contains only the information on how ArcView needs to find the data in your computer, and how to assemble such data.