[GRASS5] [bug #3302] (grass) cs2cs with GRASS and NTv2 grid fails

[Paul Kelly via RT]

this bug's URL: http://intevation.de/rt/webrt?serial_num=3302

Request number 3302 was commented on by 'pkelly' (Paul Kelly). 
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Cc: grass5 at grass.itc.it

Yes the reason $GISBASE/etc/nad/ doesn't need to be included in the nadgrids 
definition in the PROJ_INFO file is that the functions which prepare the 
GRASS co-ordinate system definitions for use by PROJ (pj_get_kv() and 
pj_get_string() in lib/proj/get_proj.c) call the PROJ function pj_set_finder
() to specify a name of a "finder function", which is provided by GRASS 
(actually set_proj_lib() in get_proj.c; should really be re-written using 
G_asprintf() if it is staying) and returns the location of the datum 
gridshift files in $GISBASE/etc/nad/. PROJ then calls this function whenever 
it wants during the transformation if it needs to find the files.

At the time we were putting the datum transformation support into GRASS, 
Frank indicated that this was the preferred solution.
But looks like we've hit a limitation.

My peferred solution would be, as long as specifying the full path to the 
gridshift file is supposed to work and isn't an undocumented feature of 
PROJ, would be to change pj_get_kv() (which is called by g.proj) to prepend 
$GISBASE/etc/nad/ to the name of the gridshift file when it prepares the 
PROJ string, and to do away with pj_set_finder().



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