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

[Jan Hartmann]

That's clear, thanks for the information.

I have two questions:

- Is there an error factor available for each data point, i.e. how 
precise are the grid shifts really?
- Are the original measurements available, to try out different 
interpolations?

Just curious for the German situation, I'm going to ask the same 
question at the Dutch topographic service. There is no ntv2 file 
available for our country, and I would like to know what happens if I 
try to create one with this method, unknow to them :-)

Jan

On 01/22/2013 03:38 PM, Schmitz, Uwe wrote:
> Jan,
>
>> 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?
>>
> I was involved in the process of creating the
> BeTA2007 grid as a member of the (AdV-)working group.
>
> So let me try to shed some light on this.
>
> The resulting BeTA2007 grid is (as Hermann mentioned)
> *not* created using one standalone interpolation/transformation
> method.
> It's an amalgamation of the most suitable sub-grids
> used by the individual federal states (Bundeslaender).
> Each subgrid has been calculated by using the method
> which was preferred by the indiviual state.
> The subgrids were then "composed" to a country
> wide grid, using a special algorithm to average
> the differences at state boundaries.
>
> Not enough, there have been additional constraints to
> preserve. So it's not unusual that a grid, computed
> based on some arbitrary chosen test data, has nothing
> to do with something obtained through the above
> outlined process.
>
> The test data is only meant for verifying
> implementations and to ensure, that an unknown
> software is doing a good job ;-)
>
> Hope that helps a little bit in understanding
> BeTA2007.
>
> Uwe
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