Getting GPS Data Into Your GIS System

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Getting GPS Data IntoYour GIS System

• Jeff Grussing
• Leader, GIS Development

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Topics for Discussion

• 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
  Satellite-Based 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 users
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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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.

Ionosphere
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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

• 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

• 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
• 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
• 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

• Television
• There are roughly 2,800 television antennas
  around the U.S. These wont 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 Rosums 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 Europes 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 dont have existing digital mapping or
  data, its 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 8743'41"
• 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

• ESRIs 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 GPSData Into Your GIS
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Using ArcPAD to GetGPS 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!