[OSRS-PROJ] cs2cs and datum shift]

[Roberto Vidmar]

Frank,

Frank Warmerdam wrote:

...snip

> 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...

...snip

> Good luck,
>
Thanks for your help.

Roberto

-- 
------------------------------------
                                                             
Roberto Vidmar 
  ___  __  __    OCEANOGRAFIA
 /  / /__ /__    GEOFISICA
/__/ /__/ __/    SPERIMENTALE
                                   
Istituto Nazionale di Oceanografia e
di Geofisica Sperimentale
34010 Sgonico (TS) ITALY

Research Group: CARS
(CArtography and Remote Sensing)

Tel. +39 040 2140 336 direct
Tel. +39 040 2140 1  operator

 E-mail: rvidmar at inogs.it
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