[GRASS-dev] [bug #4877] (grass) d.grid -g, ps.map: incorrect geogrid rendering

[Paul Kelly]

Hello Hamish

On Wed, 19 Jul 2006, Request Tracker wrote:

> this bug's URL: http://intevation.de/rt/webrt?serial_num=4877
> -------------------------------------------------------------------------
>
> Subject: d.grid -g, ps.map: incorrect geogrid rendering
>
> Hi,
>
> "d.grid -g" and ps.map's geogrid command are not calculating the correct grid
> lines for me. (lines are drawn 90m to the north)
>
> Perhaps it is only doing the projection, and not the datum transform ?!!?

Looking at the code, this seems quite possible. I have applied a patch in 
CVS - can you test? If it's working now, I expect we should be able to 
apply a similar patch to ps.map.

Slightly more in-depth explanation:
this behaviour stems from a questionable "feature" of PROJ (my words 
really although Frank has agreed too---see PROJ bug 368 
http://bugzilla.remotesensing.org/show_bug.cgi?id=368 ) where if one 
co-ordinate system has datum transformation parameters specified and 
the other does not, a kind of partial transformation is done. IMHO if the 
datum information is not fully specified it should be ignored completely, 
but that is not possible the way the datum information is contained as 
part of the co-ordinate system in PROJ. Thierry Laronde and KerGIS were 
doing this a different way to avoid this problem as far as I remember...

Paul