avoiding the gps pitfalls indiana gis conference

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  • Avoiding the GPS PitfallsIndiana GIS Conference

  • Avoiding the Common GPS Pitfalls:

    GPS Best Practices

    Lanny Schnipper

    Mapping Sales Manager


  • 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?

  • Equipment Setup

    Turn off WiFI and Bluetooth to conserve battery.

    Automobile charger and serial clip for charging.

    External battery pack or spare battery

  • Current 2008 WM6.1Geo OS and GPS FW Start/Settings/System/System Information

  • Current Geo 6000 WM6.5 OS and GPS FW Start/Settings/System/System Information/GNSS

  • Know your hardware Specs

  • Antenna Height:Internal or External

  • 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.

  • Best Practices: In the field

    1 second logging Averaging

  • Open conditions: Good for Sub-Foot to Centimeter Horizontal

  • MultipathOccasionally 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 Antennas have better multipath rejection (Stealth technology).

  • Collect good GPS measurements between features

    Collect good quality data before and after you collect your feature (maintain GPS/Initialization - lock).

    In theory, you dont 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.

  • Log Now or LaterLog 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.

  • 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.

  • Log GPS again if not satisfied with accuracy

  • Planning Sky plotView DOPS and future satellite constellation, including trajectories and trails.Use time in the field more efficiently Increase productivityIn Field Planning

  • Trimble Planning Onlinehttp://www.trimble.com/GNSSPlanningOnline

  • Best Practices: In the fieldAvoid 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, USAFrankfurt, GermanyChristchurch, New Zealand

  • Be Mindful of Buildings or other obstructions

    Wait for stable GPS Initialization and Carrier lock.

    Use offset measurements or Laser Rangefinder.

  • 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.

  • GPS Global Positioning SystemGlonass GLObal Navigation Satellite SystemGNSS 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)

  • http://incors.in.gov/map.htmlhttp://incors.in.gov/default.aspx

  • Cellular VRS ConnectionMust 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.

  • Post ProcessingBase Files vs. Base Provider?

  • Post Processing GNSS Data

  • Datum ShiftKnow 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

  • Im in NAD83NAD83 (CORS96)DatumsChoosing 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)

  • Understanding a GPS Datasheet

  • Estimated AccuraciesBased on:- Receiver type- DOP valueSNRsNumber of satellites- Satellite elevation angleCarrier lock time / initialization (ProXRT and GeoXH)Correction source (WAAS, Beacon or VRS)In the Field vs. Post Processed estimates - baseline

  • Accuracy Definitions

  • 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

  • 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/

  • Jay Riester

    Mapping Tech Support




    **Trimble offers a wide variety of GPS options that span the range of accuracy needs.*********This slide illustrates the next step Confirming the GPS reference Frame. Knowing which reference frame can be the difference between a meter plus and mean life or death to your otherwise complacent GIS manager.CEP Circular Error of Probability based on the 50% Confidence LevelRMS Root Mean Square based on the 63-68% Confidence Level (Range of 63-68% due to there being 2 dimensions N and E). Strict 1-Sigma 1-dimension probability would be 68% Confidence. 2D RMS Two Degrees Root Mean Square based on the 95-98% Confidence Level (Range of 95-98% due to there being 2 dimensions N and E).

    Trimble specifies accuracies in terms of RMS (63-68% confidence level) in open sky environments (no multipath sources). TerraSync and PFO also display Estimated Accuracies in terms of RMS which means that 63-68% of the time the actual position should be within the estimated distance of the measured position (assuming no multipath error). If a GPS position was collected in an open environment, and has an estimated accuracy of 1 meter there is a 63-68% probability that the true position is within 1 meter of that GPS position.PPMDepending on Hardware and software we can be more precise. Using the correct Datum to be more Accurate.

    but accuacy is "the nearness to the truth...."

    Precision is the repeatability of a measurement...so your example...is precise by not worth a hoot for accuracy.

    Discuss Seiler operations and what we do..*