[Proj] Help with use of swisstopo NTv2 grid shift ?

Sandro Santilli strk at keybit.net
Mon Jan 23 11:44:40 EST 2012

Hello everyone, 
I'm trying to understand how to use the swisstopo grid shift 
CHENYX06a.gsb file and would like to ask if anyone of you used
it already.

Following instructions found on this message:

I've prepared a couple of definitions pretending that the target
(CH1903+) was a WGS84 and defined the input (CH1903) as needing
the CHENYX06a.gsb gridshift:

  +proj=longlat +ellps=WGS84 +nadgrids=CHENYX06a.gsb +no_defs

  +proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs

But when trying to convert, proj4 reports that the gridshift file is only
valid within the following range of coordinates:

NTv2 CHENyx06 661x313: LL=(5.55,45.4666667) UR=(11.05,48.0666667)
pj_apply_gridshift(): failed to find a grid shift table for
                      location (759183.2731993dW,187561.6170195dN

This surprised me as the .asc version of the gridshift seems to 
contain a different range:

 S_LAT     163680.000000
 N_LAT     173040.000000
 E_LONG    -39780.000000
 W_LONG    -19980.000000

Which matches the CH1903 and CH1903+ coordinate systems, being supposedly

 DATUM_T CH1903+

Is it possible that the .gsb file is different from the .asc file ?
Or where does proj4 get the LL/UR data from ?

Any help with using CHENYX06a.gsb with proj4 is appreciated,
Thanks in advance.


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