[Proj] *SOLVED*: Creating a nadgrid shift file with GDAL

[Jan Hartmann]

I forward this mail from Hermann Peifer, about the differences between 
the official German grid shift file, and the one computed by gdal 
utilities based on 31 test points. It's a matter of different 
interpolation methods. Can anyone comment on this pretended accuracy of 
the official grid shift files, and about the interpolation methods used, 
in Germany or elsewhere?

Jan


On 01/22/2013 11:30 AM, Hermann Peifer wrote:
>
> As far as I can see: the agreement between "myshift.gsb" and 
> BETA2007.gsb is only in the immediate neighbourhood of the 31 test 
> points. For these points, the differences are below .5 millimeter.
>
> For the interpolated areas in between, there is no real agreement 
> between "myshift.gsb" and BETA2007.gsb. Which is no surprise, as the 
> official data might be interpolated using different methods and to my 
> knowledge, the interpolation has also been done differently for the 
> ex-GDR area compared to the area of former West-Germany.
>
This is important.What were those methods? The official grid-shift file 
pretends to a certain accuracy. How valid is that pretension, when 
different interpolations result in such different results? And two 
different interpolations are certainly inacceptable. Mind again: the 
pretended accuracy is several decimals behind the point. This doesn't 
make any sense, neither scientific nor in real world geodesy, if the 
interpolation method is that instable.

Jan



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