[OSRS-PROJ] cs2cs and datum shift]

Roberto Vidmar rvidmar at ogs.trieste.it
Mon Aug 26 10:28:58 EDT 2002


Frank Warmerdam wrote:


> Overall, I don't know why you can't get the correct output value, but I
> imagine it is because a different mechanism is being used by IGM to 
> express
> the 7 parameters than PROJ.4 is expecting.  PROJ.4's 7 parameter datum 
> shift
> code is based on NIMA Geotrans, and apparently matches the 7 parameter 
> parameters
> as expected for method 9606 in the EPSG database.  However, there are 
> many other
> ways of expressing the transformation.

This is not a problem. Basically having translation, rotation and scale 
means trying only 8 different parameter sets. The problem is that I 
cannot convert back again to the starting point, even using a 3 
parameter transformation.

At present I'm playing with the source code (so I must go deep inside 
the problem, deeper than I supposed and wanted to do...). I'm trying to 
understand if the problem is in the to/from WGS84 functions or in the 
geodetic/geocentric one.

Trying a wgs84 to ellps=intl with +towgs84=0,0,0 and back again restores 
latitude and longitude with a small error in height (1 cm) so I suppose 
the trick is hidden in the WGS84 functions...


> Good luck,
Thanks for your help.


Roberto Vidmar 
  ___  __  __    OCEANOGRAFIA
 /  / /__ /__    GEOFISICA
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Istituto Nazionale di Oceanografia e
di Geofisica Sperimentale
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