Transcript Chapter Five Georgraphic Information

Police Technology
Chapter Five
Geographic
Information
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
Learning Objectives
Understand:
 Basic parts of a map and how they are
used
 Theory of trilateration in conjunction with
time difference on arrival (TDOA)
 How global positioning satellites operate.
 Difference between a map and a
geographic information system
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
Introduction
For law enforcement,
geography refers to:
 Distance between two
objects
 Physical characteristics
of those objects
Screen capture provided by Pen-Link™ Inc.
Could be natural terrain,
streets, locations or
even people
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
GIS Basics
A combination of:



Computer hardware
Computer software
Data
Which enables people to create visual
displays of information
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
GIS Basics
GIS is a relational database designed to
help store, organize, and use spatial
information.
GISs are a combination of technologies
that enhance our understanding of
geography.
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
Cartography Basics


Cartography is
the science of
map making.
A map is a visual
representation of
a defined space.
Orientation
 Grid lines
 Latitude
 Longitude
 Waypoint

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Justice, Raymond E. Foster
Cartography Basics

An area of
California -
Latitude
Longitude
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster


Trilateration is the foundation
of most Automatic Vehicle
Location technologies.
Trilateration is similar to
triangulation
If you are 25
miles from Joe’s
House, you are
somewhere in the
circle
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
Trilateration
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
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
Trilateration
Add the distance to
Jane’s house, and
you are where all
three circles overlap
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
Global positioning satellites

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
Global positioning satellites (GPS) use
trilateration with a second math formula to
provide users with location.
A GPS system uses an RF receiver and a
network of satellites.
Final component is a ground control station:


Four monitoring stations
One control station.
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
Global positioning satellites


1978, U.S. launched
twenty-four satellites
in orbit. By 1994,
they were
operational.
These completed the
Dept. of Defense
Navigation Satellite
Timing and Ranging
(NAVSTAR) system.
Drawing courtesy of the United States Navy
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
Global positioning satellites
The GPS system uses trilateration and
Time Difference on Arrival (TDOA) to
give a precise location.
 Satellite orbits are predictable
 Their probable location is known in
advance
 Info on prediction of orbital path is in an
almanac (a software database)
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
Time Difference on Arrival (TDOA)


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Radio waves travel the speed of light – a
constant.
By measuring the time it takes a radio
wave to reach a receiver we can calculate
the distance from the source to the
receiver.
With three signals (three satellites) we can
use trilateration to determine location
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
Global positioning satellites
The job of the monitoring
stations and the master
control stations is:
 Track satellites
 Detect deviations from
predicted orbital paths
 Recalculate orbital path in
cases of deviation
 Update almanac info
every six hours
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
Global positioning satellites
Possible errors in GPS:
 Orbital deviation (satellites affected by Earth’s
gravity)
 Clock drift – corrected by TDOA
 Atmospheric conditions – GPS transmissions
affected by ionosphere and troposphere.
 Department of Defense (selective availability)
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
Back to GIS
Geographic Information Systems are
used by police agencies to analyze crime.
GISs use one of two alternative methods:
 Universal transverse mercator (UTM)
 State plane coordinates
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
Universal Transverse Mercator


UTM divides the world in 60 zones,
each containing 6 degrees of
latitude.
The location of an object is
measured in meters from one of the
corners of the zone and the
longitude
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
State Plan Coordinates
A system that divides
the U.S. into more
than 120 zones.
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
Spatial Data
In a GIS system, the features (objects) of a
map become spatial data. We can:
 Analyze info about a location
 Specific info about the object itself
 Analyze information about its relationship
to other features mapped in the GIS
system.
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
GIS Basics
Three kinds of data contained in a
GIS system: Points, Lines, and Areas
A GIS system combines points lines
and areas to map complex objects like
buildings, roads, or political
jurisdictions.
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
GIS Basics
Final feature is areas or polygons.
 Areas are defined by a continuous
boundary that surrounds a certain
feature (zip codes, city limits, etc.)
 A discrete site (a specific location)
can be a park, mall, or housing
project.
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
GIS Basics
What makes GIS different from traditional
maps is the ability to add information about
the features (tabular data) for analysis.
 Attribute – traits or quality of a feature
(color, number of ports, size of units, etc.)
 Value – a predefined answer that limits
possible responses to attribute.
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
GIS Basics
Image Data is a third type of data found in
maps created with GIS technology.
 Aerial photographs
 Scanned photographs
Can link tabular data (parolee address info)
and image date (photographs of parolees)
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
Getting the Data
GISs do not contain maps or graphics.
They create visual representations of
relational databases.
 Events entered into a GIS database
must be geocoded.
 Geocoding is the process of
determining the proper coordinates of
an event’s location (usually an
address)
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
Getting the Data

Basic street address
information for GIS
applications can be
obtained from both:
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Commercial sources
Government sources

Data on crimes and
specific location is
usually input by the
agency.
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
GIS Basics
Final part of GIS process is the
application of GIS tools for display
and analysis of the database.
 Pin maps
 Using different symbols for different
types of events
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
Automatic Vehicle Location
Law enforcement agencies
using GPS for
AVL can improve the
accuracy of the GPS
system with differential
correction.
 Stationary source of GPS
receiver’s location
 Compare signal
 Remove distortion
Photograph provided by Woodcrest Vehicle Center
Copyright 2005 - 2009: Hi Tech Criminal
Justice, Raymond E. Foster
Police Technology
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Justice, Raymond E. Foster