[gdal-dev] Force application of ntv2 grid

[Thomas Campagne]

2013/12/10 stevejking <stevejking at talktalk.net>

> 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
> >
> > _______________________________________________
> > gdal-dev mailing list
>
> > gdal-dev at .osgeo
>
> > http://lists.osgeo.org/mailman/listinfo/gdal-dev
>
> 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
>
>
Hi,

Just as a side note, I like to turn on the debugging mode by setting the
environment variable PROJ_DEBUG to any number.
It gives useful info on how your ntv2 grid is used and might help you
identify other issues if any.

Regards

Thomas


-- 
Thomas Campagne
Vancouver, BC
Canada
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