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NOAA Datums Videos 2014

Differential correction with NA2011 and the newest epoch of NAD83

See our and Datums Basics and GPS Tutorials for the long-awaited results of months of rigorous testing and peer review

Trimble Positions software has replaced Trimble GPS Analyst in ArcGIS 10.1

ALASKA GPS and GIS users - check out Joel Cusick's Datum and Projections presentation Feb 2013

ArcGIS 10.1 - New and Cool improvements see Presentation from Melita Kennedy, ESRI : Projections and Datums Dec 2012

Home GPS Tutorials Demo # 4: Export with 7-parameter WGS84 - NAD83 transformation
Demo # 4: Export with 7-parameter WGS84 - NAD83 transformation PDF Print E-mail

To complete our systematic analysis, our last demonstration uses NAD 1983 (2011), a 7 parameter WGS84-to-NAD83(2011) transformation for export.

A. Settings

  • Copied  N1-N7 project folders (“N” for null) to C1-C7 (“C” for Copy)
  • All .cor files exported with 7-parameter WGS84-to-NAD83(2011) transformation

            Note, one could argue these should be exported with NAD83(CORS96) but for the sake of our systematic

            analysis, choose NAD1983 (2011) for all exports”

  • Resulting .shp files assigned UTM NAD83 projection files (.prj)

 


Export: Choose Reference System and Transformation

·         Lat/Long - ITRF00 or UTM - NAD83 as shown in chart

·         NAD 1983 (2011) transformation for all.  Since N4 and N6 are already NAD83. Choosing a 7-parameter transformation shifts these locations twice –

i.e. once while differentially correcting from the NAD83 base station coordinates, and again via the WGS84-to-NAD83 transformation.  This is ok for now since we are systematically applying the transformations to all data to watch the results.

     

     

           


To view transformation pararmeters:

Utilities / Other / Coordinate System Manager /   Transformations tab/ 7- parameter transformation/  NAD 1983(2011)

B. View results in ArcMap



C. Analysis

The 3 groupings are as expected based on the “meters from base”.

C2/C3/C5

The previous “ITRF00” locations, have shifted 1.23 m southeast to the NAD83 location when exported with the NAD 1983(2011) transformation

·         C2 shifted from “HTDP-ITRF00” to NAD83(2011)

·         C3 shifted from IGS08 to NAD83(2011)

·         C5 shifted from ITRF00 to NAD83(2011)

C1/C7 and C4/C6

These 4 locations have all shifted southeast to the new location when exported with the NAD 1983(2011) transformation.  Corrected positions in the .cor files were NAD83; they took on this reference when differentially corrected against NAD83 base station.  During export, the NAD 1983(2011) transformation was applied a second time.  While this double transformation is a common error that is easily made by many, it should be avoided.

D. Conclusions


Not recommended

·         C1/C7/C4/C6 can be dismissed since the resulting locations are due to applying an ITRF-to-NAD83 transformation twice as discussed above. 

·         C2 is definitely not recommended since “HTDP-ITRF00” introduces an unpredictable, location-dependent IGS08-ITRF08 shift discussed in demo # 2.

·         C5 is not recommended since it relies on “Old” coordinates based on the relative antenna position.  NGS no longer supports or updates these positions since NA2011 released a new standard, the absolute antenna position.

“Best options”

C3 and N4 are the “best options” in light of the NA2011 and recent PFO changes but they are still not the "right way".  Consider these “temporary” work-arounds until a true solution can be found.  A desired solution is suggested  at the end of this series of pages.