[GRASS5] datum conversion problems

[Roger Miller]

Folks,

This is a little long, but please stay with me.

I am working on a project in which I need to convert data locations measured 
in UTM coordinates using NAD 27 to UTM coordinates using NAD 83.  The 
resulting coordinates are used to select an elevation from a digital 
elevation model in UTM coordinates using NAD 83 with a 10-m spacial 
resolution and a vertical resolution of 0.1 meters.  The results I produce 
need to be reproduceable by other parties using other software and any 
differences between our results must be negotiated.  Potentially millions of 
dollars can swing on a systematic error.

So far the results are not reproduceable and negotiations are progressing.

I have long regarded the conversion programs from the National Geodetic Survey 
(NGS) as being authoritative.  The Corpscon program released by the Army 
Corps of Engineers produces results that are identical to results from the 
NGS software.  I think Corpscon is little more that an GUI for the NGS 
software.  The nad2nad program that accompanies the Proj package produces 
results that are within about 1 cm of the results from the NGS software.

The s.proj (Grass 5.3) and v.proj (Grass 6.0.1) programs produce results that 
are systematically different from the results of the NGS program.  In my data 
set, the eastings differ by a very consistent 13 meters.

About 3 years ago I adapted the grass [rsv].proj programs to do the nad27 <-> 
nad83 conversion.  I recall that at that time I benchmarked the process 
against the NGS software.  Subsequently the [rsv].proj modules were modified 
to perform more general transformations using the pj_transform function in 
the Proj library.  The pj_transform function returns results that are 
different from the results returned by nad2nad, or the NGS software.

My first question is probably more appropriate for the Proj developers, but 
I'll ask it here first, hopefully avoiding the need to join Yet Another 
Mailng List.  Which provides the more correct result for converting nad27 to 
and from nad83, nad2nad.c or pj_transform?

My second question regards CVS.  I wanted to reconstruct the old 5.0 versions 
of v.proj so that I might review the method I used and confirm its results.  
The HTML gateway to CVS shows information for the old version, but that 
program refers to functions that aren't present in the code or in the proj 
library.  Could that code have been lost from CVS?

I am concerned by what appears to me to be an avoidable inaccuracy in GRASS, 
but my problems are small compared to my cooperators problems.  They are 
using ESRI software and are unable to reproduce the NGS results within wide 
margins.  I asked a friend of mine who regularly uses ESRI software to 
benchmark the results against the NGS software.  She used a few different 
methods and found that the nad83 locations reported by the ESRI ArcEditor 
software were not close to the NGS results.  Differences were as large as 100 
meters.

The ESRI software is closed source, so it is impossible for my cooperators to 
defend their results -- something they are painfully aware of.


Roger Miller