geographic information systems. what is a geographic information system (gis)? a gis is a particular...

Geographic Information SystemsGeographic Information Systems

What is a Geographic Information What is a Geographic Information System (GIS)?System (GIS)?

• A GIS is a particular form of Information System applied to geographical data.

• An Information System is a set of processes, executed on raw data to produce information which will be useful when making decisions.

• A system is a group of connected entities and activities which interact for a common purpose.

This discussion is derived from a seminar by Dr. David Waits


• An information system has a full range of functions to:– process observations– process measurements– provide descriptions– explain data– make forecasts– make decisions


• In a geographic information system, information is characterized spatially.

• In a GIS the common purpose is decision making to manage: – land– resources– transportation– retailing– OR any other spatially distributed activity


• A GIS is an organized collection of computer hardware, software, geographic data, and personnel to efficiently capture, store, update, manipulate, analyze, and display all forms of geographically referenced information.

• A GIS integrates spatial and other kinds of information within a single system to provide a consistent framework for analyzing geographic (spatial) data.


• A GIS makes connections between activities based on geographic proximity.

• The digital data structure can be conceptualized as a set of “floating electronic maps” with a common registration allowing the used to “look” down (drill down) and across the stack of maps.

• The spatial relationships can be summarized (data base inquiries)


• The spatial relationships can be summarized (data base inquiries) or manipulated (analytical processing).

• Another definition of GIS - An internally Another definition of GIS - An internally referenced, automated, spatial information referenced, automated, spatial information system for data mapping, management, system for data mapping, management, and analysisand analysis

CaptureData

GIS ProcessGIS Process

RegisterMap Base

InterpretData

Convert Datato DigitalFormat

Store Datain Computer

ProcessData

DisplayResults

GIS GIS SystemSystem

SpatialSpatialDataDataBaseBase

AttributeAttributeDataDataBaseBase

CartographicCartographicDisplay SystemDisplay System

Geographic Analysis System

Map Map DigitizingDigitizingSystemSystem

ImageImageProcessingProcessing

SystemSystem

StatisticalStatistical Analysis Analysis System System

DatabaseDatabaseManagementManagement

SystemSystem

ImagesImages

MapsMaps

MapsMaps

StatisticalStatisticalReportsReports

StatisticsStatisticsTabular DataTabular Data

GIS - Map StackingGIS - Map StackingGIS - Map StackingGIS - Map Stacking

GeographicGeographicInformationInformationSystemSystem

Courtesy of PPI

NDVI From Aerial Image

pH Layer

Nitrogen AvailabilityEstimate fromAerial Photo

““Drilling Down” Through The Data LayersDrilling Down” Through The Data Layers

Courtesy of PPI

GIS Data FormatsGIS Data Formats

• There are two formats used by GIS systems to store and retrieve geographical data:

– Raster

– Vector

Raster FormatRaster Format

• Data are divided into cell, pixels, or elements

• Cells are organized in arrays• Each cell has a single value• Row and Column Numbers are used to

identify the location of the cell within the array.

• Perhaps the most common example of raster data is a digital image.

Vector FormatVector Format

• Data are associated with points, lines, or boundaries enclosing areas

• Points are located by coordinates• Lines are described by a series of

connecting vectors (line segments described by the coordinates of the start of the vector, its direction, and magnitude or length).

• Areas or polygons are described by a series of vectors enclosing the area.

Vector FormatVector Format

• Any number of factors or attributes can be associated with a point line or polygon.

• Data are stored in two files:– a file containing location information– a file containing information on the attributes

• A third file contains information needed to link positional data with their attributes.

Vector and Raster Representation Vector and Raster Representation of of Point Point Map FeaturesMap Features

Map FeatureGIS Vector

FormatGIS Raster

Format

(X,Y)Coordinate in space

Cell Locatedin an Array

Vector and Raster Representation Vector and Raster Representation of of LineLine Map Features Map Features


FormatGIS Raster

Format

Vector and Raster Representation Vector and Raster Representation of of AreaArea Map Features Map Features


FormatGIS Raster

Format

Vector and Raster FormatsVector and Raster Formats

• Most GIS software can display both vector and raster data.

• Raster formats are efficient when comparing information among arrays with the same cell size.

• Raster files are generally very large because each cell occupies a separate line of data.

• Vector formats are efficient when comparing information whose geographical dimensions are different.

Comparison of Raster and Vector Comparison of Raster and Vector FormatsFormats

• Raster formats are efficient when comparing information among arrays with the same cell size.

• Raster files are generally very large because each cell occupies a separate line of data, only one attribute can be assigned to each cell, and cell sizes

are relatively small.

• Vector formats are efficient when comparing information whose geographical shapes and sizes are different.

• Vector files are much smaller because a relatively small number of vectors can precisely describe large areas and a many attributes can be ascribed to these areas.

RasterRaster VectorVector

Comparison of Raster and Vector Comparison of Raster and Vector FormatsFormats

• Raster representations are relatively coarse and imprecise

• Vector representations of shapes can be very precise.

RasterRaster VectorVector

Most GIS software can display both raster and vector data. Only a limited number of programs can analyze both types of data or make raster type analyses in vector formats.

Coordinate SystemsCoordinate Systems

• Spatial data are generally recorded as latitude and longitude, frequently as decimal degrees.

• Other systems commonly used are the Universal Transverse Mercatur - UTM and State Plane Coordinates. These systems are projections of the curved surface of the globe on to a plane surface.


• UTM, the preferred system, distance unit is the meter.

• The unit of the state plane system is the foot.

• There is generally a different coordinate system for each state in the state plane system.

• In the UTM system projections are made in zones of approximately 6 degrees of longitude.


• There are two datums (reference planes) commonly used to make projections: North American Datum of 1927 (NAD27) and the World Geographic Reference System of 1984 (WGS84). The WGS84 datum can be used world wide. The default datum of many GPS receivers is the WGS84 datum.

UTM ZonesUTM Zones

LongitudeRange Zone

78-84 17 N

84-90 16 N

90-96 15 N Tulsa

96-102 14 N Stillwater

102-108 13 N

108-114 12 N

114-120 11 N

UTM SpecificationsUTM Specifications

• UTM position is specified by:– Number of the Zone– North (or South) of the equator– East of the western boundary of the zone– Distances are in meters

• Coordinates are referred to as “Northings” and “Eastings”– N xxxxxx, E yyyyyy

Interpolation to Predict Missing Interpolation to Predict Missing DataData

• Frequently, data are collect at discrete points located a significant distance apart or some of the data are missing.

• Interpolation is used to predict the values of the missing data.

• There a number of interpolation algorithms available in SST Toolbox and other software.

Interpolation AlgorithmsInterpolation Algorithms

• Nearest neighbor• Local Averaging• Inverse distance to a power• Radial bias functions• Shepard’s Method• Kriging

AND• Simple Contouring

What is the effect of the interpolation What is the effect of the interpolation algorithm on the estimate of missing data?algorithm on the estimate of missing data?

1 6 11 16 21 26 31 36 41 46 51 56 61 66

1

6

ppm

Distance, ft

Efaw 1x1 Phosphorus

60-70

50-60

40-50

30-40

20-30

10-20

0-10

Selected Missing Data

Nearest NeighborNearest Neighbor

• Value of the nearest measurement to the missing data.

• In the case of values at the same distance, the average of those values

Missing Data

Nearest Neighbor

Local AverageLocal Average

• Average of all values within a predetermined distance.

Missing Data

Averaged Values

Inverse (Weighted) DistanceInverse (Weighted) Distance

• Values are weighted by the inverse of their distance from the missing value. The weights can be raised to a power. The interpolated value is equal to the sum of the weighted values divided by the sum of the weights.

Missing Data

Search Radius < 3 ft

Inverse (Weighted) DistanceInverse (Weighted) Distance

W = 1

W = 0.707

W = 0.5

W = 0.447

W = 0.354

Missing Data

Missing Values and Predicted Values Missing Values and Predicted Values Phosphorus at EfawPhosphorus at Efaw

0

10

20

30

40

50

60

5 10 15 20 25 30 35 40 45 50 55 60 65

Missing Element Location, ft

Ph

op

ho

rus,

pp

m

Missing Value Nearest Neighbor Average Inverse Dist.

Error In Predicting Missing DataError In Predicting Missing Data

0

20

40

60

80

100

5 10 15 20 25 30 35 40 45 50 55 60 65

Element Location

Abs

olut

e Er

ror,

% Nearest Neighbor

Average

Inverse Distance

Comparison on Interpolation AlgorithmsComparison on Interpolation Algorithms

Nearest Neighbor

Average of Adjacent Elements

Inverse Distance

Radius<3ft

______________________________ % Error ____________________________

17.3 23.1 19.5

Prediction by Linear interpolation Between Prediction by Linear interpolation Between Every Fifth Data PointEvery Fifth Data PointEfaw 1 by ExperimentEfaw 1 by Experiment

010203040506070

0 20 40 60 80

Distance, ft

Ph

os

ph

oru

s,

pp

m

Trans 4

Fifth Data Point

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