Transcript
GPS and GIS Jeff Grussing Leader, GIS Development Mike Siedschlag GIS Technician
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Topics for Discussion • • • • • •
Understanding of how GPS works Accuracy, equipment and cost Other peripherals for GPS systems DGPS vs. Post Processing Benefits of using GPS data in GIS What are some of the misconceptions about GPS data in GIS • How do we get GPS data into GIS • Things to watch out for in GPS data • Recap SM
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 •
Human Error – Equipment Configuration/Setup, Equipment Use
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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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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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Data Collectors Trimble Data Collectors
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Data Collectors Leica Data Collectors
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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 SM
Mapping Grade Receivers
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Mapping Grade Receivers Data Collectors with integrated onboard GPS receivers
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Survey Grade GPS GPS Total Station
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GPS Hardware Cost • Mapping Grade – 2 to 5 meter Accurate GPS units range from $2,500 to $5,000 – Sub meter Accurate GPS units range from $5,000 to $10,000
• Survey Grade – Centimeter accurate GPS total stations are approximately $25,000 to $50,000
• GPS Peripherals – Beacon Receivers range from $1,500 to $2,500 – Different types of antennas cost vary depending on accuracy – Laser range finders range from $2,500 to $5,000
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Additional Antennas and Beacon Receivers • Antennas – Used to increase accuracy – Vehicle mounted – Reference location for survey grade receivers
• Beacon Receivers – Needed for real time DGPS – Some are integrated into the GPS receiver – Some are external and communicate through cable or Blue Tooth wireless connection SM
GPS Equipment Additional Antenna's and Beacon Receivers
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Other GPS Possibilities • • • •
GPS PCMCIA cards for Laptops and Notebooks GPS compact flash cards for PDA’s Low cost external USB or nine pin GPS devices Most of these devices are mapping grade 2 to 5 meter accurate. • There is some talk about a sub meter accurate PCMCIA card that is in the works and is to be released sometime this fall. SM
GPS Equipment Office Support Module which includes cradles for data collectors and batteries and also communication ports for connection to the Desktop PC
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GPS Equipment Laser Range Finders can be connected to GPS collectors for use in field data collection which can greatly improve the rate at which data is collected. The laser is aimed at the object in which you wish to obtain a GPS location for and the laser’s internal compass and distance information are the fed into the GPS receiver and a location is derived from the combination of this data and the GPS signal received by the GPS receiver.
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Mounting Options
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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 port 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 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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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. SM
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. SM
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. SM
GPS Vs. Existing Mapping Data
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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. SM
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 SM
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Getting GPS Data into GIS • 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 • • • • • • •
ESRI’s ArcPad Excel spreadsheet Access Programs provided by GPS manufacturer XML Delimited ASCII Text Almost any GIS Software SM
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. Went through GPS equipment, accuracy’s and cost. Discussed other peripherals for GPS systems. Defined DGPS and compared to Post Processing. 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. SM
Questions
Thank you!
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