[Proj] Problems using grid shift files

[Ziegler Stefan]

Hi

thanks for your answers and sorry for not replying for approx. 2 months.
I will now spend some time on my problems with grid files. I thought
first that the shift grid files can be used for any reprojection (eg.
21781 to 32632). But it's only usable for one specific reprojection
(defined in the file header). The reprojection is more an equalization
than a "simple" reprojection since the old reference frame (CH1903/LV03)
is inaccurate. By adding the +nadgrid parameter I'm able to reproject
correctly only to one reference system. Reprojections to other systems
are wrong. Is there a way to tell proj4 that it only considers the grid
file for one system?

My intention is to use the grid file for the reprojection of large
raster dataset (aerial images) in web mapping services. I can reproject
the whole dataset statically but this approach doubles the place
requirement :( (~300 gigs). And what about other data I serve via a web
mapping service? I'd like a more dynamic way. One solution would be
something like a chain of reprojections: every time I want to reproject
from 21781 to 4326 there will be a additional reprojection (21781 ->
2056 -> 4326) in the background. But I have no idea how to do this. 

Any help and other ideas are appreciated!
regards
Stefan




> -----Ursprüngliche Nachricht-----
> Von: Paul Kelly [mailto:paul-grass at stjohnspoint.co.uk]
> Gesendet am: Freitag, 23. Januar 2009 16:12
> An: PROJ.4 and general Projections Discussions
> Betreff: Re: [Proj] Problems using grid shift files
> 
> On Fri, 23 Jan 2009, Mikael Rittri wrote:
> 
> > I think cs2cs normally assumes that a grid shift file will 
> transform a national datum to WGS84.
> > So I don't know how chenyx06.gsb can be used by cs2cs, but 
> perhaps some pipe
> > construction could be used.
> 
> Specify the gridshift file as part of the PROJ.4 description 
> of the CH1903 
> system, and use a +towgs84=0,0,0 datum parameter set for the CH1903+ 
> location. Something along those lines should work I think, but the 
> co-ordinates would need to be transformed again to get to WGS84.
> 
> Paul
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