Geographic Information Systems

Copyright Protected 2005: Hi Tech Criminal Justice, Raymond E. Foster

Police Technology Police Technology Chapter FiveChapter Five

Geographic Geographic Information Information


Learning ObjectivesLearning Objectives

Understand: Understand: Basic parts of a map and how they are Basic parts of a map and how they are

usedused Theory of Theory of trilaterationtrilateration in conjunction in conjunction

with with time difference on arrival (TDOA)time difference on arrival (TDOA) How How global positioning satellitesglobal positioning satellites

operate.operate. Difference between a map and a Difference between a map and a

geographic information systemgeographic information system


IntroductionIntroductionFor law enforcement, For law enforcement,

geography refers to: geography refers to: Distance between two Distance between two

objects objects Physical characteristics Physical characteristics

of those objectsof those objects

Could be natural terrain, Could be natural terrain, streets, locations or streets, locations or even peopleeven people

Screen capture provided by Pen-Link™ Inc.


GIS BasicsGIS Basics

A A combination of: combination of: Computer hardware Computer hardware Computer software Computer software Data Data

Which enables people to create visual Which enables people to create visual

displays of informationdisplays of information


GIS is a GIS is a relational databaserelational database designed designed to to

help store, organize, and use spatialhelp store, organize, and use spatial

information. information.

GISs are a combination of technologiesGISs are a combination of technologies

that enhance our understanding of that enhance our understanding of

geography. geography.



Cartography BasicsCartography Basics

Cartography is Cartography is the science of the science of map making. map making.

A map is a visual A map is a visual representation of representation of a defined space. a defined space.

OrientationOrientation Grid linesGrid lines LatitudeLatitude LongitudeLongitude WaypointWaypoint


Cartography BasicsCartography Basics

An area of An area of California -California -

LatitudeLatitude

LongitudeLongitude


TrilaterationTrilateration is the is the foundation of most foundation of most Automatic Vehicle Location Automatic Vehicle Location technologies. technologies.

Trilateration is similar to Trilateration is similar to triangulationtriangulation

If you are 25 miles from Joe’s House, you are somewhere in the circle


If you are 25 miles from Joe’s House, and 15 miles from Bob’s House, you are somewhere in the area of the circles’ overlap

TrilaterationTrilateration


TrilaterationTrilateration

Add the distance to Jane’s house, and you are where all three circles overlap


Global positioning satellites (GPS) use Global positioning satellites (GPS) use trilateration with a second math formula to trilateration with a second math formula to provide users with location. provide users with location.

A GPS system uses an RF receiver and a A GPS system uses an RF receiver and a network of satellites. network of satellites.

Final component is a ground control station: Final component is a ground control station: Four monitoring stations Four monitoring stations One control station. One control station.

Global positioning satellitesGlobal positioning satellites



1978, U.S. launched 1978, U.S. launched twenty-four twenty-four satellites in orbit. satellites in orbit. By 1994, they were By 1994, they were operational. operational.

These completed the These completed the Dept. of Defense Dept. of Defense

Navigation Satellite Navigation Satellite Timing and Ranging Timing and Ranging (NAVSTAR) system. (NAVSTAR) system.

Drawing courtesy of the United States Navy



The GPS system uses trilateration andThe GPS system uses trilateration and

Time Difference on Arrival (TDOA)Time Difference on Arrival (TDOA) to to

give a precise location. give a precise location. Satellite orbits are predictableSatellite orbits are predictable Their probable location is known in Their probable location is known in

advanceadvance Info on prediction of orbital path is in Info on prediction of orbital path is in

an an almanacalmanac (a software database) (a software database)


Time Difference on Arrival (TDOA)Time Difference on Arrival (TDOA)

Radio waves travel the speed of light Radio waves travel the speed of light – a constant.– a constant.

By measuring the time it takes a By measuring the time it takes a radio wave to reach a receiver we radio wave to reach a receiver we can calculate the distance from the can calculate the distance from the source to the receiver.source to the receiver.

With three signals (three satellites) With three signals (three satellites) we can use trilateration to determine we can use trilateration to determine locationlocation



The job of the monitoring The job of the monitoring stations and the stations and the master control stations master control stations is: is:

Track satellitesTrack satellites Detect deviations from Detect deviations from

predicted orbital paths predicted orbital paths Recalculate orbital Recalculate orbital

path in cases of path in cases of deviationdeviation

Update almanac info Update almanac info every six hoursevery six hours


Possible errors in GPS: Possible errors in GPS: Orbital deviationOrbital deviation (satellites affected by (satellites affected by

Earth’s gravity) Earth’s gravity) Clock driftClock drift – corrected by TDOA – corrected by TDOA Atmospheric conditionsAtmospheric conditions – GPS transmissions – GPS transmissions

affected by ionosphere and troposphere. affected by ionosphere and troposphere. Department of Defense Department of Defense (selective (selective

availability)availability)



Back to GISBack to GIS

Geographic Information Systems are Geographic Information Systems are

used by police agencies to analyze used by police agencies to analyze crime. crime.

GISs use one of two alternative GISs use one of two alternative methods: methods:

Universal transverse mercator (UTM) Universal transverse mercator (UTM) State plane coordinatesState plane coordinates


UTM divides the world in 60 UTM divides the world in 60 zones, each containing 6 zones, each containing 6 degrees of latitude. degrees of latitude.

The location of an object is The location of an object is measured in meters from one of measured in meters from one of the corners of the zone and the the corners of the zone and the longitudelongitude

Universal Transverse MercatorUniversal Transverse Mercator


State Plan CoordinatesState Plan Coordinates

A system that divides A system that divides the U.S. into more the U.S. into more than 120 zones. than 120 zones.


Spatial DataSpatial Data

In a GIS system, the In a GIS system, the featuresfeatures (objects) (objects) of aof a

map become map become spatial dataspatial data.. We can: We can: Analyze info about a locationAnalyze info about a location Specific info about the object itselfSpecific info about the object itself Analyze information about its Analyze information about its

relationship to other features relationship to other features mapped in the GIS system. mapped in the GIS system.


Three kinds of data contained in a Three kinds of data contained in a

GIS system: GIS system: Points, Lines, and AreasPoints, Lines, and Areas

A GIS system combines points lines A GIS system combines points lines

and areas to map complex objects and areas to map complex objects likelike

buildings, roads, or politicalbuildings, roads, or political

jurisdictions. jurisdictions.



Final feature is areas or Final feature is areas or polygonspolygons. . Areas are defined by a continuous Areas are defined by a continuous

boundary that surrounds a certain boundary that surrounds a certain feature (zip codes, city limits, etc.)feature (zip codes, city limits, etc.)

A discrete site (a specific location) A discrete site (a specific location) can be a park, mall, or housing can be a park, mall, or housing project. project.



What makes GIS different from traditionalWhat makes GIS different from traditional

maps is the ability to add information aboutmaps is the ability to add information about

the features (the features (tabular datatabular data) for analysis. ) for analysis. AttributeAttribute – traits or quality of a feature – traits or quality of a feature

(color, number of ports, size of units, etc.)(color, number of ports, size of units, etc.) ValueValue – a predefined answer that limits – a predefined answer that limits

possible responses to attribute. possible responses to attribute.



Image Data is a third type of data found inImage Data is a third type of data found in

maps created with GIS technology. maps created with GIS technology. Aerial photographsAerial photographs Scanned photographs Scanned photographs

Can link tabular data (parolee address info)Can link tabular data (parolee address info)

and image date (photographs of parolees)and image date (photographs of parolees)



Getting the DataGetting the DataGISs do not contain maps or graphics.GISs do not contain maps or graphics.

They They create visual representationscreate visual representations of of

relational databases. relational databases. Events entered into a GIS database Events entered into a GIS database

must be geocoded. must be geocoded. GeocodingGeocoding is the process of is the process of

determining the proper coordinates determining the proper coordinates of an event’s location (usually an of an event’s location (usually an address)address)


Getting the DataGetting the Data

Basic street Basic street address address information for GIS information for GIS applications can be applications can be obtained from obtained from both:both: Commercial sourcesCommercial sources Government Government

sourcessources

Data on crimes and Data on crimes and specific location is specific location is usually input by usually input by the agency.the agency.


Final part of GIS process is the Final part of GIS process is the

application of GIS tools for displayapplication of GIS tools for display

and analysis of the database. and analysis of the database. Pin maps Pin maps Using different symbols for Using different symbols for

different types of events different types of events



Automatic Vehicle LocationAutomatic Vehicle Location

Law enforcement Law enforcement agencies using GPS foragencies using GPS for

AVL can improve the AVL can improve the accuracy of the GPS accuracy of the GPS

system with system with differential differential correction.correction.

Stationary source of Stationary source of GPS receiver’s GPS receiver’s locationlocation

Compare signalCompare signal Remove distortionRemove distortion

Photograph provided by Woodcrest Vehicle Center


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