Geographic Information Systems

Copyright Protected 2005: Hi Tech Criminal Justice, Raymond E. Foster
Police TechnologyPolice Technology
Chapter FiveChapter Five
GeographicGeographic
InformationInformation

Copyright Protected 2005: Hi Tech Criminal Justice, Raymo
Learning ObjectivesLearning Objectives
Understand:Understand:
 Basic parts of a map and how they areBasic parts of a map and how they are
usedused
 Theory ofTheory of trilaterationtrilateration in conjunction within conjunction with
time difference on arrival (TDOA)time difference on arrival (TDOA)
 HowHow global positioning satellitesglobal positioning satellites operate.operate.
 Difference between a map and aDifference between a map and a
geographic information systemgeographic information system

IntroductionIntroduction
For law enforcement,For law enforcement,
geography refers to:geography refers to:
 Distance between twoDistance between two
objectsobjects
 Physical characteristicsPhysical characteristics
of those objectsof those objects
Could be natural terrain,Could be natural terrain,
streets, locations orstreets, locations or
even peopleeven people
Screen capture provided by Pen-Link™ Inc.

GIS BasicsGIS Basics
AA combination of:combination of:
 Computer hardwareComputer hardware
 Computer softwareComputer software
 DataData
Which enables people to create visualWhich enables people to create visual
displays of informationdisplays of information

GIS is aGIS is a relational databaserelational database designed todesigned 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 ofthat enhance our understanding of
geography.geography.

Cartography BasicsCartography Basics
 Cartography isCartography is
the science ofthe science of
map making.map making.
 A map is a visualA map is a visual
representation ofrepresentation of
a defined space.a defined space.
 OrientationOrientation
 Grid linesGrid lines
 LatitudeLatitude
 LongitudeLongitude
 WaypointWaypoint

Cartography BasicsCartography Basics
 An area ofAn area of
California -California -
LatitudeLatitude
LongitudeLongitude

 TrilaterationTrilateration is the foundationis the foundation
of most Automatic Vehicleof most Automatic Vehicle
Location technologies.Location technologies.
 Trilateration is similar toTrilateration 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) useGlobal positioning satellites (GPS) use
trilateration with a second math formula totrilateration with a second math formula to
provide users with location.provide users with location.
 A GPS system uses an RF receiver and aA 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 stationsFour monitoring stations
 One control station.One control station.
Global positioning satellitesGlobal positioning satellites

 1978, U.S. launched1978, U.S. launched
twenty-four satellitestwenty-four satellites
in orbit. By 1994,in orbit. By 1994,
they were operational.they were operational.
 These completed theThese completed the
Dept. of DefenseDept. of Defense
Navigation SatelliteNavigation Satellite
Timing and RangingTiming 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) toto
give a precise location.give a precise location.
 Satellite orbits are predictableSatellite orbits are predictable
 Their probable location is known inTheir probable location is known in
advanceadvance
 Info on prediction of orbital path is in anInfo on prediction of orbital path is in 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 – aRadio waves travel the speed of light – a
constant.constant.
 By measuring the time it takes a radioBy measuring the time it takes a radio
wave to reach a receiver we can calculatewave to reach a receiver we can calculate
the distance from the source to thethe distance from the source to the
receiver.receiver.
 With three signals (three satellites) we canWith three signals (three satellites) we can
use trilateration to determine locationuse trilateration to determine location

The job of the monitoringThe job of the monitoring
stations and the masterstations and the master
control stations is:control stations is:
 Track satellitesTrack satellites
 Detect deviations fromDetect deviations from
predicted orbital pathspredicted orbital paths
 Recalculate orbital path inRecalculate orbital path in
cases of deviationcases of deviation
 Update almanac infoUpdate almanac info
every six hoursevery six hours

Possible errors in GPS:Possible errors in GPS:
 Orbital deviationOrbital deviation (satellites affected by Earth’s(satellites affected by Earth’s
gravity)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 DefenseDepartment of Defense (selective availability)(selective availability)

Back to GISBack to GIS
Geographic Information Systems areGeographic Information Systems are
used by police agencies to analyzeused by police agencies to analyze
crime.crime.
GISs use one of two alternative methods:GISs use one of two alternative methods:
 Universal transverse mercator (UTM)Universal transverse mercator (UTM)
 State plane coordinatesState plane coordinates

 UTM divides the world in 60 zones,UTM divides the world in 60 zones,
each containing 6 degrees ofeach containing 6 degrees of
latitude.latitude.
 The location of an object isThe location of an object is
measured in meters from one of themeasured in meters from one of the
corners of the zone and thecorners of the zone and the
longitudelongitude
Universal Transverse MercatorUniversal Transverse Mercator

State Plan CoordinatesState Plan Coordinates
A system that dividesA system that divides
the U.S. into morethe U.S. into more
than 120 zones.than 120 zones.

Spatial DataSpatial Data
In a GIS system, theIn a GIS system, the featuresfeatures (objects) of a(objects) of a
map becomemap 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 relationshipAnalyze information about its relationship
to other features mapped in the GISto other features mapped in the GIS
system.system.

Three kinds of data contained in aThree kinds of data contained in a
GIS system:GIS system: Points, Lines, and AreasPoints, Lines, and Areas
A GIS system combines points linesA GIS system combines points lines
and areas to map complex objects likeand areas to map complex objects like
buildings, roads, or politicalbuildings, roads, or political
jurisdictions.jurisdictions.

Final feature is areas orFinal feature is areas or polygonspolygons..
 Areas are defined by a continuousAreas are defined by a continuous
boundary that surrounds a certainboundary 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 housingcan 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 photographsScanned 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 Data
GISs do not contain maps or graphics.GISs do not contain maps or graphics.
TheyThey create visual representationscreate visual representations ofof
relational databases.relational databases.
 Events entered into a GIS databaseEvents entered into a GIS database
must be geocoded.must be geocoded.
 GeocodingGeocoding is the process ofis the process of
determining the proper coordinates ofdetermining the proper coordinates of
an event’s location (usually anan event’s location (usually an
address)address)

Getting the DataGetting the Data
 Basic street addressBasic street address
information for GISinformation for GIS
applications can beapplications can be
obtained from both:obtained from both:
 Commercial sourcesCommercial sources
 Government sourcesGovernment sources
 Data on crimes andData on crimes and
specific location isspecific location is
usually input by theusually input by the
agency.agency.

Final part of GIS process is theFinal 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 mapsPin maps
 Using different symbols for differentUsing different symbols for different
types of eventstypes of events

Automatic Vehicle LocationAutomatic Vehicle Location
Law enforcement agenciesLaw enforcement agencies
using GPS forusing GPS for
AVL can improve theAVL can improve the
accuracy of the GPSaccuracy of the GPS
system withsystem with differentialdifferential
correction.correction.
 Stationary source of GPSStationary source of GPS
receiver’s locationreceiver’s location
 Compare signalCompare signal
 Remove distortionRemove distortion Photograph provided by Woodcrest Vehicle Center

Copyright Protected 2005: Hi Tech Criminal Justice, Raymond E. Foster
Police TechnologyPolice Technology
Go to theGo to the Student ResourcesStudent Resources page atpage at
www.hitechcj.comwww.hitechcj.com

Geographic Information Systems

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