[Proj] Problems using grid shift files

[Paul Kelly]

On Wed, 1 Apr 2009, Ziegler Stefan wrote:

> 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.

Remember that we are slightly abusing the PROJ.4 API here by using CH1903+ 
as a pivot datum instead of WGS84. So the +towgs84 parameters for other 
co-ordinate systems must be adjusted so they describe a transformation to 
CH1903+, rather than a transformation to WGS84, in exactly the same way as 
we have used parameters of +towgs84=0,0,0 for CH1903+. If you have an 
accurate set of +towgs84 parameters for the CH1903+ --> WGS84 
transformation, I suspect you might be able to determine the parameters 
for your target datum to CH1903+ through some matrix multiplication (bear 
in mind that a set of +towgs84 parameters consist of rotational, 
translational and scaling components) but I'm not sure of the exact nature 
of the calculations you would need to do and I haven't tried it.

In other words, you should be able to avoid a chain of re-projections by 
chaining the datum transformations before doing the re-projection. That's 
how I understand it anyway but I could have missed something.

Paul