[gdal-dev] Force application of ntv2 grid

[stevejking]

Andre Joost wrote
> Am 29.11.2013 09:59, schrieb Jeremy Palmer:
>> Hi gdal-devs,
>>
>> I'm trying to shift shapefile data using a custom NTv2 grid file. The
>> logical source and destination coordinate systems are the same
>> (geographic coordinates EPSG:4167), I only want to shift the data.
>>
>> The type of command I'm trying to run is:
>>
>> ogr2ogr -f "Esri Shapefile" -a_srs "+proj=longlat +ellps=GRS80"
>> -t_srs "+proj=longlat +ellps=GRS80 +nadgrids=custom_grid.gsb +wktext"
>> dst_datasource_name src_datasource_name
>>
>> but the output shapefile does not get transformed...
>>
>> Is this because there is a short circuit in proj4 if the ellipsoids
>> are the same?
>>
> 
> No, you have to explicitely specify +towgs84=0,0,0,0,0,0,0 for the first 
> srs. Otherwise it is assumed that both share the same datum.
> 
> HTH,
> André Joost
> 
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Andre is on the right track. 

But, I believe you need to specify the +nadgrids=c:\custom_grid.gsb in your
source projection, then use the +towgs84=0,0,0,0,0,0,0 in your target.

You may get away without specifying any path to your gsb, not sure.


HTH
Regards,
Steve



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