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WELCOME

Paper Presented to: ESRI Survey and GIS Summit, August 2006 Sunday, August 6, 2006 San Diego, CA. Joel Cusick National Park Service Alaska Regional Office Joel_cusick@nps.gov. WELCOME. Handouts Available. Are You Really Mapping Submeter?. Revenge of the Shift. Transform:

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WELCOME

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  1. Paper Presented to: ESRI Survey and GIS Summit, August 2006 Sunday, August 6, 2006 San Diego, CA Joel Cusick National Park Service Alaska Regional Office Joel_cusick@nps.gov WELCOME Handouts Available

  2. Are You Really Mapping Submeter? Revenge of theShift

  3. Transform: NAD27 to WGS84_4 Fire Hydrant Survey Survey GPS

  4. The Wrong Place 50 feet

  5. Revenge of the Shift

  6. NAD83 NAD27-NAD83 N 61º 33’ 50.0” W 149º 44’ 21.0” 200’ – 800’ horizontal NAD27 N 61º 33’ 50.0” W 149º 44’ 21.0”

  7. The Insidious Shifts NAD83 – WGS84/ITRF00 3’ – 50’ horizontal

  8. Objective • Verify Datum Used in GIS • Confirm Reference Frame of GPS • Apply the Correct Transformation

  9. Step 1. Verify Datum in GIS • GIS as a blender • Datum Transformations • Standardize on NAD83 • Scale

  10. GIS as a Blender GPS • It’s the GIS Way NAD27 NAD83 WGS84

  11. Datums and Shifts • Most all coordinate systems are referenced to a specific datum • Mix two different datums for the same point Shift

  12. Datum Shift Errors • Datum shift errors are realized when different datums are mixed and wrong assumptions are made

  13. Datum Transformations • Formula to convert between two datums • Legacy GIS • Thousands • Move GPS data data to/from GIS • WGS84_To_??

  14. Datum Transformations • In a GIS They Work Both Ways

  15. Datum Transformations Myth • You Can’t Go Back! • Human Error is the Cause • Lack of documentation • Fundamental principle behind Mobile GIS

  16. Standardize on NAD83 • If you are using GPS data you should be in NAD83 Alaska - Part of the Continental United States since 1867! …But Still using the 1866 Spheroid

  17. What’s Wrong with NAD27 • Best fitting to just North America • Non Geocentric • Significant scale and azimuth distortion • Poorly determined relationship to the geoid

  18. Standardize on NAD83 • Dave Doyle in Alaska February 2005 • Cousin Vinny * Dave Doyle, Senior Geodesist – NGS, 2005. Alaska Survey and Mapping Conference, Anchorage Alaska.

  19. Standardize on NAD83 • Convert NAD27 to NAD83 using NADCON • Cases where NADCON is not a good idea • USFS reliance on PLGR’s from mid 1996 to 2000

  20. Unusual NAD27 Conversion Cases NAD27 to WGS84_4 vs NADCON * Andrew Lister, Research Forester, USDA Forest Service, 2005. email

  21. – www.fs.fed.us/database/gps Mark Roper

  22. Scale – Worry or Not? • GIS Data Scales • Data digitized at 1:24,000 • NMAS states - +/- 40 feet • Mapping GPS Errors • Large - Autonomous GPS (pre SA) • Mid - PLGR / Autonomous GPS Today • Small - Mapping Grade

  23. Step 1. Verify Datum in GIS • Standardize on NAD83! • Convert NAD27 to NAD83 using NADCON • Special Cases Require Care • Consider Scale of Data

  24. Step 2. Confirm the GPS Reference Frame • Which Reference Frame are you In? WGS84 /ITRF00 I’m in NAD83

  25. Datum Adjustments • WGS84 (G1150) – most current version • “Original” NAD83 = NAD83 (1986) • NAD83 (1992) • NAD83 (2002)…………. • NAD83 (CORS96) (Epoch 1997.00)

  26. Over Time Datums Have Grown Apart WGS84 = ITRF00(2cm) NAD83 ~ 1 meter NAD83 (1986) = WGS84

  27. Which Reference Frame are You In? • CORS users are in ITRF00

  28. Which Reference Frame are You In? • Since you corrected against a CORS station…

  29. Which Reference Frame Are You In? WGS84 (G1150) NAD83 (CORS96) ITRF00 = • CORS Post Processed • Autonomous GPS • WAAS • NDGPS (Beacon) • OMNISTAR (Satellite)

  30. GPS Base Station Providers • The Trick is knowing what reference frame

  31. Step 2. Confirm the GPS Reference Frame • Which Reference Frame Are You In?

  32. Step 3. Apply the Correct Transformation • Use Zero Transformations when Appropriate • If NDGPS differential • NAD83 Reference Frame • If post-process CORS, WAAS – ITRF00 Reference Frame • Transform using NAD83 (CORS96) Do Nothing! See Handout for Trimble and ArcPad Users

  33. Avoid the Shift • GIS Community • Surveyors • Software Developers

  34. Avoid the Shift • GIS Community

  35. http://www.ngs.noaa.gov/cgi-bin/ds_radius_sf.prl Avoid the Shift • GIS Community • Test Against Control

  36. Avoid the Shift • Surveyors • Communicate better in map products, digital data and reference stations

  37. Avoid the Shift • Software Developers • Standardize Syntax • Enhance Datum Interfaces

  38. Avoid the Shift • Software Companies • Standardize Syntax • Enhance Datum Interfaces • Embed Transformation Used inside Data Layers • MXGPS • www.mxgeo.com/

  39. Conclusion • Verify Datum in GIS • Confirm Reference Frame • Apply the Correct Transform

  40. Acknowledgements • Carl Beyerhelm, USFS • Ron Bisio – Trimble • Shane Clarke - ESRI • Michael Dennis, Geodetic Analysis • David Doyle, NGS • Melita Kennedy, ESRI • Andrew Lister, USFS • Mark Roper, USFS • Mel Philbrook, Trimble • Myles Sutherland, ESRI

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