[gdal-dev] ogr2ogr natgrids option ignored?

[Frank Warmerdam]

Sven Geggus wrote:
> Hi there,
> 
> looks like ogr2ogr just ignores my nadgrids projection parameters :(
> 
> Here is what I have:
> 
> ogr2ogr -s_srs '+proj=tmerc +lat_0=0 +lon_0=12 +x_0=4500000 +y_0=0 \
> +k=1.000000 +ellps=bessel +units=m +nadgrids=./BETA2007.gsb' \
> -t_srs '+proj=longlat +datum=WGS84' -f "ESRI Shapefile" out.shp in.shp
> 
> ogr2ogr -s_srs epsg:31468 -t_srs epsg:4326 -f "ESRI Shapefile" out.shp in.shp
> 
> While the latter works fine. The first version does not seem to honor the
> nadgrids Option. This is at least what strace hints me to. The command does
> not even try to open the BETA2007.gsb file.
> 
> Testing the same Parameters wirth cs2cs does work as expected and
> BETA2007.gsb _is_ honored:
> 
> echo 4450457 5487214 | cs2cs -f "%.12f" +proj=tmerc +lat_0=0 +lon_0=12 \
> +x_0=4500000 +y_0=0 +k=1.000000 +ellps=bessel +units=m \
> +nadgrids=./BETA2007.gsb +to +init=epsg:4326

Sven,

The thing to know about using PROJ.4 strings with GDAL/OGR is that GDAL/OGR
internally turns them into WKT.  Anything in a PROJ.4 string that is not
preserved in the WKT is lost - and this includes nadgrids directives for
which there is no obvious WKT analog.

One workaround is to add +wktext to the PROJ.4 definition.  This tells
GDAL to embed a complete copy of the PROJ.4 definition in the WKT as
an EXTENSION element so the original PROJ.4 string can be reconsistuted
exactly later.

Best regards,
-- 
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