Getting GPS Data Into
Your GIS System
Jeff Grussing
Leader, GIS Development
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Topics for Discussion
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Understanding of How GPS Works
Selecting GPS Equipment
Real Time DGPS and Types of Correction
Benefits of Using GPS Data in GIS
How Do We Get GPS Data Into GIS
Demo Using Access and ArcPAD
Post Processing
What Are Some of the Misconceptions About
GPS Data in GIS
• Things to Watch Out for in GPS Data
• Recap
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What is GPS?
Global Positioning System (GPS) is a SatelliteBased Navigation System made up of a network
of 24 satellites placed into orbit by the United
States Department of Defense. This System
works in any weather condition, anywhere in the
world 24 hours a day. It is free of charge.
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How GPS Works
The 24 GPS satellites circle the
earth twice a day in a very
precise orbit and transmits
signal information to earth. The
GPS receivers take this
information and use the
triangulation of three or more
satellites to calculate the user’s
exact location.
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Triangulation
Triangulation occurs when the
GPS receiver compares the
time a signal was transmitted
by a satellite with the time it
was received. The time
difference tells the GPS
receiver how far away the
satellite is. When the GPS
receives a time signal from at
least three satellites
simultaneously, an exact
location is obtained.
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Your Location
Sources of Error
• Ionosphere and
troposphere delays
– The satellite signal
slows as it passes
through the atmosphere.
The GPS system uses a
built-in model that
calculates an average
amount of delay to
partially correct for this
type of error.
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Ionosphere
Sources of Error
• Signal multipath — This occurs when the GPS
signal is reflected off objects such as tall buildings or
large rock surfaces before it reaches the receiver.
This increases the travel time of the signal, thereby
causing errors.
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Sources of Error
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Receiver Clock Errors
– A receiver's built-in clock is not as accurate as the atomic clocks
onboard the GPS satellites. Therefore, it may have very slight timing
errors.
Orbital Errors
– Also known as ephemeris errors, these are inaccuracies of the satellite's
reported location.
Number of Satellites Visible
– The more satellites a GPS receiver can "see“, the better the accuracy.
Buildings, terrain, electronic interference, or sometimes even dense
foliage can block signal reception, causing position errors or possibly no
position reading at all. GPS units typically will not work indoors,
underwater or underground.
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Sources of Error
• Satellite Geometry/Shading
– This refers to the relative
position of the satellites at
any given time. Ideal
satellite geometry exists
when the satellites are
located at wide angles
relative to each other. Poor
geometry results when the
satellites are located in a
line or in a tight grouping.
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Sources of Error
• Intentional Degradation of the Satellite Signal
– Selective Availability (SA) is an intentional degradation
of the signal once imposed by the U.S. Department of
Defense. SA was intended to prevent military
adversaries from using the highly accurate GPS
signals. The government turned off SA in May 2000,
which significantly improved the accuracy of civilian
GPS receivers.
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Sources of Error
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Human Error
– Equipment Configuration/Setup, Equipment Use
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Selecting GPS Equipment
• Data Collectors
– With integrated GPS receivers
– With external GPS receivers
• GPS Receivers and Accuracy
– Mapping grade
– Survey grade
• GPS Peripherals
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Beacon receivers
Different types of antennas
Laser range finders
Digital cameras
• GPS Hardware Cost
– Is directly related to accuracy
– Is also dependent on the peripherals
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Selecting Data Collectors
• Communication with GPS Receivers
– RS232 connector cable between the collector and receiver
– Blue Tooth wireless connection
– Data collectors with GPS receivers built in
• Other Blue Tooth Enabled Hardware that can be used as
a data collector
– Table PC
– PDA using Windows Pocket PC
– Laptops
• Communication with Peripherals
– RS232 connector cable between the collector and external device
– Blue Tooth wireless connection
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Selecting GPS Receivers
• Mapping Grade Receivers
– From sub meter to 2 to 5 meter accuracy
– Fairly low cost
– Good for GIS mapping
– Verticals are not very accurate
• Survey Grade Receivers
– From centimeter to decameter accuracy
– Very expensive
– Good where precision three dimensional
accuracy is required
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Differentially Corrected GPS
• What is DGPS?
– DGPS stands for differentially corrected GPS
• What are the advantages of DGPS?
– The GPS signal is corrected real time in the field
– No post processing required
• What are some disadvantages to DGPS?
– You may be required to carry additional equipment into the field
– Costs more
– Difficult to get a corrected signal in some areas
• Types of Correction
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WAAS
Radio Beacon
Television Frequencies
External RTCM
EGNOS
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Types of Correction
• WAAS
– Wide Area Augmentation System is a system of two Geosynchronous
satellites and 25 ground stations that provide GPS signal corrections.
• The origins of WAAS
– The Federal Aviation Administration (FAA) and the Department of
Transportation (DOT) are developing the WAAS program for use in
precision flight approaches.
• How it works
– WAAS consists of approximately 25 ground reference stations positioned
across the United States that monitor GPS satellite data. Two master
stations, located on either coast, collect data from the reference stations and
create a GPS correction message.
• Who benefits from WAAS?
– Currently, WAAS satellite coverage is only available in North America.
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Types of Correction
• Beacon
– The U.S. Coast Guard operates the most common
DGPS correction service. This system consists of
a network of towers that receive GPS signals and
transmit a corrected signal by beacon transmitters.
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Types of Correction
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Television
– There are roughly 2,800 television antennas around the U.S. These won’t
have to be modified in any way for TV-GPS to work.
– A GPS device, cell phone, laptop, PDA or other portable gadget equipped with
a Rosum TV measurement module chip picks up television signals being
broadcast in a given area, much like a typical GPS device picks up satellite
signals. From these signals it triangulates its latitude and longitude.
– Unlike the GPS system, TV stations don't have a common synchronized clock,
which is necessary to give an accurate position. So the system uses a
computerized monitor unit to track and measure TV signals.
– The location server crunches data from the monitor unit and synchronizes the
broadcast-channel clocks to determine the position of Rosum’s TV-GPS chips.
It then transmits that location data to the chips.
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Types of Correction
• Television
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Types of Correction
• External RTCM
– Radio Technical Commission for Maritime Services.
– RTCM Recommended Standards for Differential GNSS
(Global Navigation Satellite Systems) Service, Version 2.3
(RTCM Paper 136-2001/SC104-STD) – This standard is used
around the world for differential satellite navigation systems,
both maritime and terrestrial.
– RTCM Recommended Standards for Differential Navstar GPS
Reference Stations and Integrity Monitors (RSIM), Version
1.1 (RTCM Paper 137-2001/SC104-STD) – A companion to the
preceding standard, this standard addresses the performance
requirements for the equipment which broadcasts DGNSS
corrections.
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Types of Correction
• EGNOS
– European Geostationary Navigation Overlay Service.
– Is Europe’s first venture into satellite navigation. It will augment
the two military satellite navigation systems now operating, the
US GPS and Russian GLONASS systems and make them
suitable for safety critical applications such as flying aircraft or
navigating ships through narrow channels.
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Benefits of GPS System
Inventory Data in GIS
• Exact location known of electric devices.
• Ability to store spatial location as attributes in
relevant tables.
• Correct the spatial locations of facilities in the GIS
system, making them more accurate.
• With a full inventory, you have the ability to
calculate a systems worth, i.e. calculating equity
for financial and property tax purposes.
• Finding illegal foreign attachments.
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Benefits of GPS System
Inventory Data in GIS
• Can help with right of way
– Clearing
– Easements
– Planning new routes for facilities
• Also help in Operations
– Notifying customers of planned outages or Maintenance work
scheduled in their area.
• If you don’t have existing digital mapping or data, it’s a
good place to start.
• End result is better data which helps you make better
decisions.
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GPS Vs. Existing Mapping Data
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How GPS Data is Stored
• Two ways to represent geographic coordinates
(latitude and longitude):
– Decimal Degrees
• -87.728055
– Degrees, Minutes, Seconds (DMS)
• W 87°43'41"
• Either form is capable of representing the same
amount of data
• Conversions may be necessary from Decimal Degrees
to DMS or DMS to Decimal Degrees depending on:
– Format data is collected or provided
– Personal preference
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Getting GPS Data Into GIS
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ESRI’s ArcPad
Excel spreadsheet
Access
Programs provided by GPS manufacturer
XML
Delimited ASCII Text
Almost any GIS Software
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Using ArcPAD to Get GPS
Data Into Your GIS
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Using ArcPAD to Get
GPS Data Into Your GIS
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Using ArcPAD to Get GPS
Data Into Your GIS
Demo ArcPad Check in
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Using ASCII Text or Access to
Get GPS Data Into Your GIS
Demo ArcMap tools to add X,Y Data
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Post Processing
• Requires additional software
– Most GPS manufactures provide software
for post processing some GIS vendor also
provide extensions to the GIS the do post
processing.
• Usually requires internet access
– To get current correction data from known
beacons that have been surveyed in through
out the state.
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Post Processing
• How does post processing work?
– Using date and time data stored in the
tracking log in the receiver and comparing it
with correction data with the same date time
data from the closest known beacon.
– By comparing these two data sets correction
can be applied to the collected data resulting
in a more accurate point.
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Common Misconceptions of GPS
Data In Your Map
• Spatial Mismatch/Map Matching Problems
• Accurate GIS base map with less accurate GPS points.
• Accurate GPS data placed into a less accurate GIS base map.
• Accurate GPS data placed into a GIS base map that has a
scale too small to differentiate the GPS points.
• Visual representation
– Actual location may hinder ability to interpret map
• Drawbacks of GPS Data in your map
– Work expenses:
• Data collection
• Integration into GIS system
• GIS clean-up
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Things to Watch Out for in GPS Data
• Adjustment for Prime
Meridian
– Minnesota X,Y
Coordinates (-X, +Y)
• Number of decimal
places
– Need a minimum of six
decimal places for
accurate data collected
point
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Things to Watch Out for in GPS Data
• Un-Triangulated GPS Points
– Was the data collected properly when
satellite geometry was optimal?
– Has the data been post processed?
• Did you do some mission planning first?
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Recap
• Talked about how GPS works.
• Talked about the benefits of using GPS data
in a GIS.
• Mentioned misconception and drawbacks of
GPS data in a GIS.
• Described ways to bring GPS data into a GIS
system.
• Discussed things to consider before bringing
GPS data into a GIS.
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Questions
Thank you!
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