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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 # 1: HTDP Transformations
Demo # 1: HTDP Transformations PDF Print E-mail


A. Compare HTDP-transformed NGS IGS08 coordinates with CBS ITRF00 coordinates (N2) using NGS-provided velocities

Input and output locations should match

1. HTDP software

www.ngs.noaa.gov/TOOLS/Htdp/Htdp.shtml

 

2. Enter parameters

 

      IGS08 coordinates from NGS datasheet

     IGS08 velocities from NGS datasheet

Note, while velocities are expressed in m/yr on NGS datasheet, HTDP requests mm/yr

 

3. Results

HTDP output does not match Trimble CBS ITRF00 location (N2/C2)

 


B. Compare HTDP-transformed NGS IGS08 coordinates with CBS ITRF00 coordinates (N2) using HTDP-predicted velocities

Input and output locations should match

1. Enter parameters

   Same as previous step but with HTDP-predicted velocities rather than NGS provided

   values

 

2. Results

HTDP output still does not match Trimble CBS ITRF00 location (N2/C2)

 


C. Compare HTDP-transformed CBS ITRF00 coordinates with NGS IGS08 coordinates (N3) using HTDP-predicted velocities

Input and output locations should match

1. Enter parameters

   Velocity data is not provided in PFO, therefore, as in the previous step,

   HTDP-predicted velocities were used

 

2. Results

HTDP output does not match NGS IGS08 location (N3/C3).

 

 

 

 

 

 

 

 

 


D. Conclusions

Observations

Currently, base stations in the Trimble CBS list are listed such as (CORS, GARLIC (GARL), NEVADA (ITRF00 (1997)-Derived from IGS08 (NEW)).  These are NGS IGS08 coordinates have been transformed to ITRF00 using HTDP.  Entering the CBS ITRF00 coordinates into HTDP should return the IGS08 locations on the NGS datasheet.  Yet, our demonstration could not match CBS and NGS coordinates using HTDP to move from ITRF00-to-IGS08 or IGS08-to-ITRF00 using station specific or HTDP-predicted velocities.   

 

 

Explanation

This is because HTDP treats ITRF08 and IGS08 as equivalent; they are not.  IGS08 has regional adjustments that ITRF08 lacks.  These adjustments are not incorporated into Trimble’s IGS08-to-ITRF00 transformations to populate the CBS list.  Therefore, they are not truly ITRF00.  Trimble notes the ITRF00 coordinates are “HTDP-derived from IGS08”; we refer to them as “HTDP-ITRF00.” 

Significance

Transforming IGS08 epoch 2005.0 coordinates ITRF00 epoch 1997.0 for use in PFO introduces an error due to the difference between IGS08 and ITRF08.  This error is incorporated into any GPS data differentially corrected with these HTDP-transformed coordinates in the CBS.  This is unacceptable.