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Richard C. Daniels Application Platform Strategies Washington State Department of Transportation

The ISB Geographic Information Technology Standards: Accuracy Issues with Projecting Data into NAD 83 (1991) using ESRI’s ArcGIS or ArcInfo Software. Richard C. Daniels Application Platform Strategies Washington State Department of Transportation P.O. Box 47430 Olympia, WA 98504-7430

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Richard C. Daniels Application Platform Strategies Washington State Department of Transportation

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  1. The ISB Geographic Information Technology Standards:Accuracy Issues with Projecting Data into NAD 83 (1991) using ESRI’s ArcGIS or ArcInfo Software Richard C. Daniels Application Platform Strategies Washington State Department of Transportation P.O. Box 47430 Olympia, WA 98504-7430 Phone: 360-705-7799 Email: danielri@wsdot.wa.gov

  2. Why do we Care? • Washington State Information Services Board (ISB) accepted two new Geographic Information Technology Standards: • Horizontal Datum and Coordinate System • MetaData • Most State agencies are required to comply with the standards • Standards available at online http://wagic.wa.gov

  3. Who do they Apply? • Significant “data sets maintained by executive and judicial branch agencies and educational institutions, as provided by law, that operate, manage, or use IT services or equipment to support critical state business functions.” • When completed, data sets developed as part of the Washington State Geospatial Framework will be “significant geo-datasets”.

  4. What do they Require? • Minimum documentation –Metadata • Data must be stored, or readily available in the North American Datum 1983 (1991 adjustment). • Data must be in one of the following coordinate/projection system: • Washington Coordinate System of 1983 (a.k.a., Washington State Plane, zone appropriate) • NAD 83 (1991) based Geographic

  5. When will they be Implemented? • By August 30, 2003 - agencies list their significant geodata sets and send copy of list to their DIS Senior Technology Consultant • By January 30, 2004 - agencies file a copy of their conversion plan with their DIS Senior Technology Consultant • June 30, 2004 - all significant geo-datasets will be maintained or readily available on demand (within one business day) in the standard datum and coordinate system • Agency director will confirm implementation status of GIT standards in their annual IT Portfolio Update Confirmation Letter

  6. The Problem! • Issue 1, Coordinates obtained when converting from NAD 1927 Geographic Coordinates (Longitude, Latitude) to NAD 1927 Washington State Plane Coordinate System does not match those produced by equations published by the NGS • Issue 2, Coordinates obtained when converting from NAD 83 to the NAD 83 (1991) are incorrect for Stations east of 119ºW.

  7. The Projection and Transformation Process • The number of transformations required is determined by the projection and datum of both the source and desired output. • For example, to go from NAD 27 State Plane, South Zone to NAD 83 (1991) State Plane, South requires four transformations: NAD 27 SP SZ NAD 27 Geo NAD 83 Geo NAD 83 (1991) Geo NAD 83 (1991) SP SZ

  8. Issue 1, NAD 27 Geographic to NAD 27 State Plane (South): ArcGIS Vs. CORPSCON

  9. Source of Error: NAD 27 Geographic to NAD 27 State Plane • ESRI’s projection engine uses equations published in: John Snyder, 1987. Map Projections –A Working Manual, U.S. Geological Survey Professional Paper 1395 • The official source for these equations is the National Geodetic Survey and is published in: Charles Claire, 1986. State Plane Coordinates by Automatic Data Processing, Coast and Geodetic Survey Publication 62-4. • The NGS publication contains zone dependent constants to force the equations to fit look-up tables used by the Coast & Geodetic Survey prior to 1962. These constants were not used by Snyder.

  10. Issue 1, Solutions or Workarounds • Do nothing, few GIS users are impacted by a shift of 0.1 m (0.3 ft) • Don’t project or store data in a single NAD 27 State Plane coordinate system zone • Create a new State Centered plane coordinate system…

  11. Issue 2, NAD 83 to NAD 83 (1991): ArcGIS Vs. CORPSCON Note, if you converted data from NAD 27 State Plane to NAD 83 HARN you must consider both sources of error.

  12. NAD 83 and HARN • High Accuracy Reference Network (HARN) is a local adjustment of the NAD 83 datum developed by the U.S. National Geodetic Survey (NGS) • A HARN is derived based on physical survey observations from a High Precision Geodetic Network (HPGN) • A State may contain one to many HARNs. • The 1991 adjustment of the Washington-Oregon HARN is referred to as NAD 83 (1991) –Note, the current NGS adjustment is NAD 83 (1998). • In ESRI terminology, NAD 83 (1991) = HARN = HPGN

  13. Projection Process for ArcGIS and ArcInfo • Determine the number of transformations needed • Find the HARN grid that the to-be-projected-point falls into • Transform the coordinates using the selected • HARN grid and • algorithms published by the NGS (e.g. NADCON or CORPSCON)

  14. NAD 83 (1991) Projection Issue: • When projecting Coverages, TINs, or Grids to NAD 83 (1991) using ArcToolBox or Workstation, the incorrect HARN grid may be used since the first HARN grid found that contains the to-be-projected-point is used in the projection. • In the case of Washington this means that the West Idaho-Montana HARN, instead of the Washington-Oregon HARN, is used for data east of 119° W • Results in position errors up to 1 meter.

  15. Accuracy Error: HARN Grids

  16. Precision Error: NGS Algorithms European Petroleum Survey Group (EPSG) Geodesy Parameters, 31 July 2002

  17. Summary of Error Sources NAD 27 SP SZ 0.10 m (Issue 1) NAD 27 Geo 0.15 m (Precision) NAD 83 Geo 1.00 m (Issue 2) NAD 83 (1991) Geo 0.05 (Precision) NAD 83 (1991) SP SZ _____________ Maximum Conversion Error 1.30 m (4.3 ft)

  18. Issue 2, Solutions or Workarounds • Do nothing, only for data with minimal accuracy requirements (Source Data 1:24K or Smaller) • For Data with Stringent Accuracy Requirements (1:1 to 1:24K) • Import the Coverage, Tin, or Grid into a Geodatabase or Shapefile • Project using the tools in ArcToolBox • Export as back to Coverages as needed • Method used should be documented in the Metadata

  19. Help! My data is already in NAD 83 (1991), what should I do? Using the Project command at the Arc Prompt (or ArcToolBox) project your data back to NAD 83. Option 1. Follow the methods previously discussed, Or Option 2. Temporarily replace ArcInfo’s out of the box HARN grid lookup tables on your system with two custom lookup tables developed by ESRI; then you can directly project Coverages, Tins, or Grids back to NAD 83 (1991) using ArcInfo or ArcToolBox.

  20. Thanks to: WAGIC White Paper: • http://wagic.wa.gov/Techstds2/NAD83ProjectionPaper2.doc • Updated HARN Projection Files may be obtained from • Joy Paulus, ESRI, (360) 754-4727 x8954 Frank Fisher, WDNR Tim Young, WDFW Greg Tutor, WDNR & Melita Kennedy, Jeff Barrette, & Joy Paulus, ESRI

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