avoiding the gps pitfalls indiana gis conference
TRANSCRIPT
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Avoiding the GPS PitfallsIndiana GIS Conference
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Avoiding the Common GPS Pitfalls:
“GPS Best Practices”
Lanny Schnipper
Mapping Sales Manager
http://seilermapsupport.wordpress.com/
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Equipment Setup
• Current OS and GPS Firmware installed.
• Current Date and Time on the data collector.
• Data collector is fully charged.
• Software Installed, compatible and working?
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Equipment Setup
• Turn off WiFI and Bluetooth to conserve battery.
• Automobile charger and serial clip for charging.
• External battery pack or spare battery
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Current 2008 WM6.1Geo OS and GPS FW
Start/Settings/System/System Information
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Current Geo 6000 WM6.5 OS and GPS FW
Start/Settings/System/System Information/GNSS
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Know your hardware Specs
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Antenna Height:Internal or External
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Best Practices: In the field
• Horizontal/Vertical and In the Field/Post Processed accuracy settings.
• Post Processing Distance from Base.
• Estimated Accuracy icon on the status bar (click on it to display) TerraSync and GPS Correct.
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Best Practices: In the field
• 1 second logging – “Averaging”
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Open conditions: Good for Sub-Foot to Centimeter Horizontal
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Multipath
Occasionally multipath errors occur
• GPS Signal is being delayed/disrupted. “Chain Link Fences, Automobiles, Metal & Glass buildings and bodies of water.”
• Estimated error may not always include multipath.
• Multipath tends to cancel out when you collect GPS measurements while moving.
• External Antenna’s have better multipath rejection (Stealth technology).
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Collect good GPS measurements between features
– Collect good quality data before and after you collect your feature (maintain GPS/Initialization - lock).
– In theory, you don’t have to collect too many positions in a point feature, once the accuracy indicator readings are good enough.
– But if you collect for about 20 to 30 seconds, you increase the chance of acquiring more satellites or cancelling out short-term multipath errors.
– If the accuracy indicator readings are poor, you should log more positions or cancel and start again.
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Log Now or Later
• Log Now – start a feature and simultaneously start collecting GPS position.
• Log Later – start a feature, enter attributes and start collecting GPS positions later. Use this for
lines and areas too.
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Pause GPS logging before entering attribute information:
– Allows you to concentrate on positioning:• Holding the antenna over the location to
be measured.• Holding the antenna horizontal with a good
view of the sky.
– It is not easy to do this while concentrating on entering attribute information.
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Log GPS again if not satisfied with accuracy
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Planning Sky plot View “DOPS” and
future satellite constellation, including trajectories and trails.
Use time in the field more efficiently
Increase productivity
In Field Planning
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Trimble Planning Online
http://www.trimble.com/GNSSPlanningOnline
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Best Practices: In the field
– Avoid blocking satellites with your own body use an external antenna.
– The majority of satellites are between you and the equator (GPS only).
– Be aware of surrounding Obstacles.
Denver, USA Frankfurt, Germany
Christchurch, New Zealand
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Be Mindful of Buildings or other obstructions
– Wait for stable GPS Initialization and Carrier lock.
– Use offset measurements or Laser Rangefinder.
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Trees and canopy
– Use an external antenna on a pole for greater GPS gain and stability.
– Wait for a stable lock before entering canopy environments.
– Collect data before and after entering canopy environments (carrier data that is free of cycle slips).
– Use offset measurements or Laser Rangefinder.
Light Canopy
Heavy Canopy
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• GPS – Global Positioning System• Glonass – GLObal Navigation Satellite
System• GNSS – Global Navigation Satellite System –
multiple GPS Satellite Systems: Europe and China are building their own now.
– If you are using a GPS unit that can track and record GPS and Glonass you can increase your productivity tougher GNSS conditions.
– Use a RTN/VRS.– Post Process against a GNSS base
station!!!!
GPS and Glonass (GNSS)
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http://incors.in.gov/map.htmlhttp://incors.in.gov/default.aspx
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Cellular VRS Connection• Must have cellular DATA service in
your work area:– Verizon, AT&T, US Cellular,T-
Mobile or Sprint etc.– Data Plan is needed (differs from a
voice plan)
• A simple cell phone with a data plan is work best. Avoid Blackberry and Iphone.
• Make sure “Tethering” or “PAM” is enabled for Bluetooth
• Carry external battery pack to charge phone or bring a spare battery. Car charger for GPS Unit and phone.
• A Bluetooth cell phone uses less battery then a MiFi, Droid WiFI etc.
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Post ProcessingBase Files vs. Base Provider?
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Post Processing GNSS Data
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Datum Shift• Know your
correction source.
• WAAS and Beacon are different! WGS84 vs. CORS96
• Confirm your base station if post-processing.
• Find one base station that works and stick with it for post processing.
• Compare certified known control monument(s) before starting a project
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I’m in “NAD83”
NAD83 (CORS96)
Datums
• Choosing the wrong datum can greatly shift your data!
• NAD27 to NAD83 could mean 50 feet or more Horizontal.
• “NAD83” has different versions:– NAD83 (86)– NAD83 (91)– NAD83 (97)– NAD83 (CORS 96)– IGS08 (epoch 2005)
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Understanding a GPS Datasheet
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Estimated Accuracies
Based on:- Receiver type- DOP value-SNR’s-Number of satellites- Satellite elevation angle-Carrier lock time / initialization (ProXRT and GeoXH)-Correction source (WAAS, Beacon or VRS)-In the Field vs. Post Processed estimates - baseline
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Accuracy Definitions
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Test on a known control point
•Find a NGS Control via Critigen for Android Ap:•http://www.critigen.com/mobile/findacontrol/
•NGS Data Sheet via DS World/Google Earth:•http://www.ngs.noaa.gov/PC_PROD/PARTNERS/index.shtml
•NGS Data Sheet online: •http://www.ngs.noaa.gov/cgi-bin/datasheet.prl
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Other Resources:
•Trimble Knowledge Network: http://www.trimble.com/tkn/
•GPS-The-First-Global-Satellite-Navigation-System-by-Trimble. Great book on the History and GPS terms:
http://ebookbrowse.com/gps-the-first-global-satellite-navigation-system-by-trimble-pdf-
d230204802
•Seiler Mapping Support Blog: http://seilermapsupport.wordpress.com/