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<font face="Times New Roman, Times, serif">That's clear, thanks for
the information.<br>
<br>
I have two questions:<br>
<br>
- Is there an error factor available for each data point, i.e. how
precise are the grid shifts really?<br>
- Are the original measurements available, to try out different
interpolations?<br>
<br>
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 :-)<br>
<br>
Jan<br>
<br>
</font>
<div class="moz-cite-prefix">On 01/22/2013 03:38 PM, Schmitz, Uwe
wrote:<br>
</div>
<blockquote
cite="mid:1A8DF958A1AA1E46BDDB48C08CB0DD003252DCAB@MAIL-V.bezreg-koeln.nrw.de"
type="cite">
<pre wrap="">Jan,
</pre>
<blockquote type="cite">
<pre wrap="">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?
</pre>
</blockquote>
<pre wrap="">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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</pre>
</blockquote>
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