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

Jan Hartmann j.l.h.hartmann at uva.nl
Tue Jan 22 06:14:25 EST 2013

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?


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.


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