Spatial -

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 # 3: “Seed” NGS coordinates and Export with null transform
Demo # 3: “Seed” NGS coordinates and Export with null transform PDF Print E-mail

This demonstration mutes the need for this comparison by manually entering base station coordinates directly from NGS datasheets.

A. Settings

  • New project folders 3-7 were created with the same .ssf file as N1/N2
  • .cor files exported with NAD83(Conus) null transformation
  • .shp files assigned ITRF00 or UTM NAD83 projection files (.prj) as appropriate

Choose Reference Position Source

Base provider (2nd option) offers the opportunity to manually enter coordinates

1. Select the same CORS station

2. Click Copy

3. On the General tab, for Organization enter a name that describes the CORS site, source and reference system

4. On the Base Station tab, edit to match NGS

    New L1 IGS08 IGS08 coordinates and altitude

5. Click OK and proceed with the differential correction

Export: Choose Reference System and Transformation

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

·         Null transformation for all. 

Since base station coordinates are NAD83, the GPS data shifts to NAD83 during differential correction.  By choosing  NAD83(Conus), the same null transformation used in Demo # 2, one avoids shifting the GPS data a second time during export.

B. View results in ArcMap

1. Start a new ArcMap document containing no projection information

2. Since a NAD83 data frame is desired, add the expected NAD83 locations N4 and

    N6 first

3. Verify the map projection is UTM NAD83

4. Verify the spatial reference for N4 and N6 is UTM NAD83

5. Add the expected ITRF00 locations: N1, N2, N3, N5 and N7.

    Since these ITRF00 locations do not match the NAD83 map, a transformation to

    align these layers in ArcMap can be specified.

6. Click the Transformations button

7. Choose to convert from NAD83 to WGS 84 using the _5 transform

    Note: Same results in ArcGIS 10 as shown or in ArcGIS 10.1 with either [ITRF2000 to WGS84] +

   [WGS84(ITRF00) to NAD83 transform or [ITRF2000 to WGS84] + [NAD83 to WGS84_5] transform

8. Verify spatial reference for N1, N2, N3, N5, N7 is ITRF00

9. Verify the map projection is still UTM NAD83

    For N1, N2, N3, N5 N7 the data remains stored in ITRF00 but has been projected

   “on the fly” in ArcMap to match the current NAD83 data frame.


C. Analysis

Demo # 2 showed the 1.23 m difference between the “ITRF” and “NAD83” locations reflects the “meters from base distance”.  This corresponds to the difference between NAD83 and ITRF00 in this area.  Below 2 groupings are one might have predicted from the “meters from base” distances.


·         N2 and N5 are within 10 mm of each other

The difference between locations of the N5 (NGS ITRF00) and N2 (“HTDP ITRF00”) reflects location-specific IGS08 adjustments that were lost when the NGS IGS08 positions were HTDP-transformed to ITRF00.

·         N3 is 15 cm from N5/N2

This is the difference is the datum shift between IGS08 (N3) and ITRF00 (N5/N2).


·         N1 and N7 locations are one and the same.  This is not surprising as N1 coordinates were supplied by Trimble and the same coordinates were seeded for N7.  

·         The N6 location within a mm of N1/N7

·         The N4 location is slightly shifted from N6 reflects the new absolute antenna position compared to the older antenna position and the datum shift between and NAD83(2011) and NAD83(CORS96)

D. Conclusions

Unacceptable -

·         N2(HTDP-ITRF00)/N3(IGS08)/N5(ITRF00)/ can be dismissed since these do not meet the original goal of a NAD83 export. 

·         N6 is not good since it relies on “Old” coordinates which are based on the previous relative antenna standard that NGS no longer supports.  The same could be said of N5.

·         N7 is not good since these same coordinates are already the base file.  Re-entering them introduces the element of human error.  Re-read the results of demo # 1 before you insist on using the N1 workflow.

Best option in light of changes

In light of these Sept 2011 release of NA2011 and recent changes in PFO, N4 is the one of the best workflows above all others tested because the base coordinates were –

·         manually entered from well-documented source, the NGS datasheet

·         reflect the new absolute antenna position,

·         are in the latest epoch of NAD83, NAD83(2011)

·         meet our goals for export to GIS in a NAD83 shapefile

As Joel Cusick, NPS points out, “this is still not recommended as the ‘right way’, since this is really a ‘temporary’ work-around until a true solution can be found.”