[Proj] Problems using grid shift files

[Ziegler Stefan]

Dear List

we have some problems using grid shift files. We are using a grid shift
file which was developed by the Swiss Federal Office of Topography [1].
We just duplicated epsg 21781 (to 921781) and 2056 (to 902056) in the
epsg file and added  "+nadgrids=chenyx06.gsb" and "+nadgrids=@null":

21781: 
+proj=somerc +lat_0=46d57'08.66"N +lon_0=7d26'22.50"E +ellps=bessel
+x_0=600000 +y_0=200000 +towgs84=674.374,15.056,405.346 +units=m +k_0=1
+no_defs 

2056:
+proj=somerc +lat_0=46d57'08.66"N +lon_0=7d26'22.50"E +ellps=bessel
+x_0=2600000 +y_0=1200000 +towgs84=674.374,15.056,405.346 +units=m
+k_0=1 +no_defs

921781: 
+proj=somerc +lat_0=46d57'08.66"N +lon_0=7d26'22.50"E +ellps=bessel
+x_0=600000 +y_0=200000 +towgs84=674.374,15.056,405.346 +units=m
+nadgrids=chenyx06.gsb +k_0=1 +no_defs 

902056:
+proj=somerc +lat_0=46d57'08.66"N +lon_0=7d26'22.50"E +ellps=bessel
+x_0=2600000 +y_0=1200000 +towgs84=674.374,15.056,405.346 +units=m
+nadgrids=@null +k_0=1 +no_defs

4326:
+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs

904326:
+proj=longlat +ellps=WGS84 +datum=WGS84 +nadgrids=@null


Here are the results:

21781 -> 2056: correct!
921781 -> 90256: correct (with grid shift file)!

but:
921781 -> 2056: wrong!
921781 -> 4326: wrong!
921781 -> 904326: wrong!

Is the grid shift only applied if the destination definiton has a
"+nadgrids=@null"? But then why is the transformation to 904326 not
correct? Has this something to do with this bug report [2]? Or I'm just
mixing up some parameters? Same results with Proj 4.5 and 4.6.1. Any
help is appreciated.

regards
Stefan



[1]:
http://www.swisstopo.admin.ch/internet/swisstopo/en/home/topics/survey/l
v03-lv95/chenyx06/distortion_grids.html
[2]: http://trac.osgeo.org/proj/ticket/22



Mit freundlichem Gruss
Stefan Ziegler
Leiter Aufsicht

Kanton Solothurn
Bau- und Justizdepartement
Amt für Geoinformation
Rötistrasse 4
4501 Solothurn
Telefon 032 627 75 96
Telefax 032 627 75 98
stefan.ziegler at bd.so.ch
http://www.so.ch

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