[GRASS-user] Reprojection from/to gnomonic projection two years later - SOLVED

[Milan Salek]

Hello,

thanks to Paul Kelly and Frank Warmerdam (http://proj.maptools.org/), the below
mentioned issue is solved; when I simply added: "nadgrids:@null" to the
projection definition file (PROJ_INFO), then I obtained correct results. Cited
from Frank's mail:

"The trick to having cs2cs (or anything based on pj_transform()) apply no
change to coordinates when going from the sphere to WGS84 is to add
+nadgrids=@null to your gnomonic (or spherical projection) definition."

Milan


Milan Salek wrote:
> Hello,
> 
> I am referring to the reprojection problem of GRASS6.x described here:
> http://www.intevation.de/rt/webrt?display=History&serial_num=3630
> 
> To sum up, the GRASS version 5.3+ (i.e., the current version 6.2, too; I'v not
> tested CVS version) incorrectly reprojects the maps between coordinate system
> using gnomonic projection (using only sphere as a reference body) and any other
> coordinate system with projection using ellipsoid. There is obvious erroneous
> north-south shift of approx 0.19 degree (at 50 deg latitude), very likely due to
> the different reference body (and mayby because of  the mix of of
> geodetic/geometric latitude - ?).
> 
> The system proj (http://proj.maptools.org/) works OK, there is not a problem.
> 
> The old [vr].proj in GRASS 5.0.3 is reprojecting perfectly, without any shift,
> obviously handling the reference body correctly. I think that there could be
> some possibility to fix the problem if this old version works OK. Maybe the
> problem could have been connected with the introduction of the datum
> transformation function in grass5.3 (http://grass.itc.it/devel/roadmap.php).
> 
> I would like to ask if there is a possibility to fix the shortcoming.
> As a workaround, I still have to maintain the old version of GRASS 5.0.3 at one
> computer just for the reprojections using gnomonic projection. For some
> reprojection tasks I have available external programs, but the correction of the
> above mentioned error would be very helpful. If the correction of the error (or
> feature?) :o) is not planned in the future, let me know, too, please.
> 
> BTW, is anywhere a repository of old GRASS 5.0.3 for cygwin? I'v found only
> GRASS 5.4.
> 
> Thanks in advance for any response.
> 
> Milan Salek
> 


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