GIS is an information system that allows for capture, storage, retrieval, analysis and display of spatial data.

Note that GIS is closely tied to the infrastructure that exists for creating spatial data – satellites, census protocols, weather stations, land survey techniques, and more.

GIS definition

Characteristics of GIS?

GISGIS

Computer

Software

Data PeopleComputerHardware

Geographic Data

• Features must be referenced to some real world location

• Known as georeferencing

What is Geodesy?

More simply, geodesy is the study of the Earth’s size and shape.

The Earth is:

A Spheroid An Ellipsoid

Geoid -- The surface on which gravity is the same as its strength at mean sea level

Types of projections

(a) Azimuthal (b) Cylindrical (c) Conic. Light source positions, also called perspective positions, play an especially important role in planar projections. (Different aspects, such as polar, equatorial, or oblique, will also affect the map

projection)

Tangent vs. Secant Projections

Standard line

Standard lineStandard line

standard point/lines: on a projected map, the location(s) free of all distortion at the exact point or lines where the surface (cylinder, cone, plane) touches the globe.

latitudepositive in n. hemispherenegative in s. hemisphere

longitudepositive east of Prime

Meridiannegative west of Prime

Meridian

Geographic Coordinate System

• This is a Global Coordinate System• Based on angles on the ellipsoidal Earth

Cartesian Coordinates

Computationally, it is much simpler to work with Cartesian coordinates than with spherical coordinates

x,y coordinatesreferred to as “eastings” & “northings”defined units, e.g. meters, feet

Transverse Mercator Projection applied to each 6o zone to minimize distortion

UTM Zone Projection

Universal Transverse Mercator

• Each half has a different y-origin:

• For the Northern half of the zone, the y-origin is at the Equator– Northing at equator is

0m• For the Southern half of

zone the y-origin is 10,000,000 meters south of the Equator – Northing at equator is

10,000,000m

UTM• The central meridian, which runs

down the middle of the zone, is used to define the position of the origin

• Distance units in UTM are defined to be in meters, and distance from the origin is measured as an Easting (in the x-direction) and a Northing (in the y-direction)

• The x-origin is west of the zone (a false easting), and is placed such that the central meridian has an Easting of 500,000 meters

UTM vs SPCS

• SPCS– More accurate than UTM– used primarily for engineering applications, e.g. utility

companies, local governments to do accurate surveying of facilities network (sewers, power lines)

– Used for small areas– Difficult to use over larger areas (when multiple zones

are necessary)

• UTM– allows overlap between zones for mapping purposes– The UTM system is global

Terminology

• Data Object -- digital representation of an entity

Pasture

Road

House

Review

• The relationships between data objects in space.

Topology

• Entities in the real world are represented as one of the following in a GIS:– Vector data:

• Points• Lines• Areas (or polygons)

– Raster data• Pixels in an array

2 GIS Data Models

Key concept!

Cell (x,y)

The raster data model represents the Earth’s surface as a two-dimensional array of grid cells, with each cell having an associated value:

1 2 3 5 8

4 6 8 3 9

3 5 3 3 1

7 5 4 3 9

2 2 4 5 2

Cell value

Cell size = resolution

columns

row

sRaster data model (details later)

Vector Data Objects

Geographic building blocks

• Points

– 0 dimensional

• Lines

– 1 dimensional

• Polygons

– 2 dimensional

Thought question:

How are you going to represent the California OAK tree in digital format?

A point? A polygon? Or a pixel?

It will depend on:- Scale of observation- Purpose of your research- The type of data you have access to in the GIS

Topology: Connectivity

10 11 12

13 14

15

⑤

① ②

③

④

Arc From-Node To-Node1 10 112 11 123 11 134 13 155 13 14

Arc-node list

Connected arcs are determined by searching through the list for common node numbers.

Because of the common node 11, arcs 1, 2, and 3 all intersect. The computer can determine that it is possible to travel along arc 1 and turn onto arc 3. But it is not possible to turn directly from arc 1 to arc 5.

Topology: Contiguity

Two geographic features which share a boundary are called adjacent. Contiguity is the topological concept which allows the vector data model to determine adjacency.

An ArcFrom-Node To-Node

Direction

left

right

BC

D

E

1

2

3

4

5

67

8

9

Arc Left Right Polygon Polygon5 C B9 E C10 ? ? 1 ? ?

A line = a series of connected cells that portray length

Problems with this representation? • Line may be narrower than pixels show.• e.g. Road layer: how much of a road needs to be in a cell in order to be considered a ‘road’ cell?

Raster Data Model - Lines

Area = a group of connected cells that portray a shape

What problems could we have with this representation?• What if a lake’s edge falls in a pixel?

Raster Data Model - Areas

Cell Size & Resolution• The size of the cells in the raster data

model determines the resolution at which features can be represented

• The resolution can have an effect on which features are represented in what locations:

10 m Resolution 1 m Resolution5 m Resolution

Rules for Assigning Cell Values•Cell values can be assigned to cells accorded to some set of rules, and selecting those rules differently can also effect the representation of features:

10,10,10, 100, 100, 4, 1, 4, 0,20, 4, 1, 4, 0,20, 2, 1, 6, 0,20, 2, 1, 6, 0,20, 2, 1, 6, 0,20, 2, 1, 6, 0,20, 100, 10

There is a tendency towards spatial autocorrelation; for nearby cells to have similar values - values often occur in runs across several cells

11100

10000

10000

00000

00000

00111

00111

00111

00000

00000

00000

00000

00111

00111

00111

00000

00000

11100

11100

11100

Raster Data Storage – Run Length Encoding

This approach takes advantage of patterns in the data, taking advantage of the repetition of values in a row:

45 values

row by

row

header

Vector Data Model - Advantages• It is a good representation of the world as we see it

(our visual systems automatically segments the world we see by identifying objects)

• The topology of a layer can be fully described and explicitly stored

• It is efficient in terms of data storage– It only uses storage for objects of interest and

does not need to store values for the spaces in between

• No jaggy edges (raster has these on any diagonal)• Useful for network analysis and modeling flows of

linear features

Vector Data Model - Disadvantages

• The data structure is more complex

– especially when you have fully encoded topology (i.e. using the arc-node model)

• It is more difficult to write computer programs to manipulate data

• Spatial analysis operations can be more difficult

Raster Data Model - Advantages• The data structure is much simpler

• It is easy to overlay and combine layers

• This model better represents continuous data

• Raster data is easily integrated with satellite (and other remotely-sensed) data

• Writing programs to manipulate raster is easier

• It is easy to do simulation modeling due to uniform size and shape of grids (i.e. it is easy to define uniform modeling units)

Raster Data Model - Disadvantages

• Because a value must be stored for each and every cell in a grid, there is a great deal of redundancy and large storage requirements

• Location can be captured only as accurately as the resolution allows, which is determined by the cell size

• Spatial analyses that are based on topological relationships are not well supported by this model

Intended use

• Scale of your data must be chosen based on:

1. Your data needs

2. The intended use of the existing data

Row = object

Column = property

Vector setting: attribute data:

Cell (x,y)1 2 3 5 8

4 6 8 3 9

3 5 3 3 1

7 5 4 3 9

2 2 4 5 2

Cell value

columns

row

s

Raster attributes: cell values

Attribute data

Three types of data:

• Nominal scale

• Ordinal level

• Interval and ratio levels

Spectral Resolution of Landsat TM

TM Band: 1 2 3 4 5 7

These bands provide a coarse summary of spectral signatures.

Remote Sensing

• Remote sensors are devices that sense energy from a remote location (i.e., a device not in physical contact with what it is sensing)

• Remote sensing is the science of acquiring, processing and interpreting information/data collected by remote sensors.

Remote Sensing

• Active – emit energy and detect reflections– Sonar– Radar– Lidar

• Passive - detect emitted/reflected energy from other sources– Satellite sensors– Air photos– Cameras– Video recorders

Satellite Imagery• Digital data is obtained by sensors on satellite

platforms.

Satellite Imagery

• Described by five resolutions– Spatial resolution: area on ground represented by each pixel

• Advanced Very High Resolution Radiometer (AVHRR) – 1 km• Landsat - 30m• SPOT – 2.5m - 20m / 2.5m - 10m• IKONOS - 1m/4m

– Temporal resolution: how often a satellite obtains imagery of a particular area

– Spectral resolution: specific wavelength intervals in the electromagnetic spectrum captured by each sensor

– Radiometric Resolution: number of possible data values reportable by each sensor (how sensitive the sensor is to changes in brightness of objects that it views)

– View angle resolution: the number of angles at which the ground objects are recorded by the sensor.

Multispectral Imagery Display

BLUEBLUE

GREENGREEN

REDRED NEAR IR SHORT

WAVE IRMID-

WAVE IRLONGWAVE IR

1Landsat TM Band = 2 3 4 5 7 6

Band Combination = 7 4 2

Color Guns =

Band Composite Output =

Color composite imageColor Composite Image

Band A Band B Band CBlue color gun Green color gun Red color gun

DeforestationEvidence of Tropical Forest Change

MSS (80 m) 19 June 1975 MSS (80 m) 1 August 1986 TM (30 m) 22 June 1992

RondoniaBrazil

Change DetectionMount St. Helen’s National Monument Park

Landsat TM 9/30/87

IRS LISS-2 9/6/94 The two left images (one from 1987 and the other from 1994) were merged for Change Detection, to show revegetation patterns since the 1980 eruption of Mt. St. Helen. The resulting image (above) shows places where revegetation has taken place (in green) and places where timber has been harvested (in red). Gray areas indicate no change.

Flood Assessment

Landsat TM (SWIR, bands 7-4-2)

Damage Assessments

Extent of damage is assessed from imagery… tornado path in yellow

Buildings with structural damage are identified and outlined

Contrast Enhancement example:• A linear stretch is one of the most common types of

contrast enhancement• Minimum BV is remapped to 0• Maximum BV is remapped to 255

0 255127

0 25560 108 158

Scattering

Scattering occurs when particles or large gas molecules present in the atmosphere interact with and cause the electromagnetic radiation to be redirected from its original path.

sun

The sky is blue because of Rayleigh Scattering. Shorter wavelengths (i.e. blue) of the visible spectrum are scattered more than longer visible wavelengths.

So why isn’t the sky indigo or violet?

Atmospheric Windows

Those areas of the spectrum which are not severely influenced by atmospheric absorption and thus, are useful to remote sensors, are called atmospheric windows.

Geometric Correction

• Four Basic Steps of Rectification1. Collect ground control points (GCPs)

2. “Tie” points on the image to GCPs.

3. Transform all image pixel coordinates using mathematical functions that allow “tied” points to stay correctly mapped to GCPs.

4. Resample the pixel values (BVs) from the input image to put values in the newly georeferenced image

Cartographic Communication

• Success or failure on the part of the cartographer depends on whether or not the map communicates the intended information.

Uses of Maps

• To record and store information for reference.

• To analyze locational distributions and spatial patterns.

• To present information and communicate findings (often to help facilitate decision-making).

Thematic vs. Reference

• Thematic maps– spatial distribution of particular variables– Often created by GIS analysts

• Reference maps– USGS Topo maps, navigation charts, tax

maps– Generally created by large mapping

organizations

Map Elements

Legend

Scale

Text:Projection/sourceNorth ArrowInset

Map/figure

Neat lineBorder

Title

Shape• Differences in forms of symbols

• Options:– abstract (e.g circle, square) – iconographic (e.g. airplane, flag, schoolhouse)

• Caution: too many different shapes can get very busy and difficult for the map reader to discriminate

Ordinal Ratio Nominal

Bad Bad Good

UtilityUtility

Hierarchical Organization

• Subdivisional– Portrays the internal divisions of a

hierarchy– Example: Regions of North Carolina

WesternMountains

PiedmontCoastal Plain

Chloropleth

Source: http://www.gis.psu.edu/geog121/pop.html

U.S. Census Data The U.S. Census acquires data from households about:

• Demographic info•Race•Population•Disabilities•Migration

• Economic info•Living quarters•Occupation•Income

• Social info•Languages spoken•School enrollment•Family structure in the home•Marital status•Ancestry•Foreign-born population

• TIGER/Line files:– The “geography” of the census

• TIGER designed to: • support pre-census functions in preparation for Census

of Population and Housing • support census-taking efforts • evaluate success of the Census • provide geographic framework for analysis of Census

data

U.S. Census Data & TIGER/Line Files

TIGER line and address data• Roads

– attributes include basic road type, address ranges