[gdal-dev] Where is the projection info stored in a GDAL dataset?

[Even Rouault]

Joaquim,

ogr2ogr -t_srs passes the CRS object as an argument of the 
GDALDataset::CreateLayer() API

GDALDataset::SetProjection() / GDALSetProjection() is (at least in all 
implementations of in-tree drivers) reserved for the raster API, and 
raster drivers that support the Create() API.

Similarly GDALDataset::GetSpatialRef() / GDALGetSpatialRef() is for the 
raster API

For the vector API, use OGRLayer::GetSpatialRef() (different layers in a 
same dataset could have different CRS)

Best regards,

Even

Le 28/03/2021 à 19:57, Joaquim Manuel Freire Luís a écrit :
>
> Hi,
>
> I am trying to use the GDAL shared lib directly from Julia and there 
> are things that keep breaking my head. For example, the lines below 
> use a mix of GMT and GDAL wrapper functions calls that *WORK*. First 
> command creates a GMT dataset with two UTM points type with an 
> assigned coordinate system. Next, in 2), I create a GDAL dataset of a 
> LineString, convert it to geog in 3) and save it to file using the 
> OGR_GMT driver in 4)
>
> 1) D2 = mat2ds([588977.324434907 4095339.69117669; 579551.921920776 
> 4150721.71333039], proj="+proj=utm +zone=29");
>
> 2) ds = gmt2gd(D2);
>
> 3) ds2=ogr2ogr(ds, ["-t_srs", "+proj=longlat", "-overwrite"])
>
> 4) ogr2ogr(ds2, dest="lixo2.gmt")
>
> Now the troubles. When I tried to use GDALSetProjection() I got an 
> error like this
>
> ERROR 6: /vsimem/##269: Dataset does not support the SetSpatialRef() 
> method.
>
> So I found that I could set the CRS with GDALDatasetCreateLayer() 
> while creating a layer on the dataset (using the Memory driver)
>
> (https://github.com/GenericMappingTools/GMT.jl/blob/master/src/gdal_utils.jl#L254 
> <https://github.com/GenericMappingTools/GMT.jl/blob/master/src/gdal_utils.jl#L254>)
>
> But oddly trying to inquiry the **ds** with GDALGetSpatialRef() 
> returns an empty pointer. I.e.:
>
> julia> Gdal.GDALGetSpatialRef(ds.ptr)
>
> Ptr{Nothing} @0x0000000000000000
>
> However the CSR was well sored in some place because the ="lixo2.gmt” 
> is correct (it has this)
>
> # @VGMT1.0 @GLINESTRING
>
> # @R-8.1/-8/37/37.5
>
> # @Jp"+proj=longlat +datum=WGS84 +no_defs"
>
> # @Jw"GEOGCS[\"unknown\",DATUM[\"WGS_1984\",SPHEROID[\"WGS 
> 84\",6378137,298.257223563,AUTHORITY[\"EPSG\",\"7030\"]],AUTHORITY[\"EPSG\",\"6326\"]],PRIMEM[\"Greenwich\",0,AUTHORITY[\"EPSG\",\"8901\"]],UNIT[\"degree\",0.0174532925199433,AUTHORITY[\"EPSG\",\"9122\"]],AXIS[\"Longitude\",EAST],AXIS[\"Latitude\",NORTH]]"
>
> # FEATURE_DATA
>
> > 
>
> -8 37.0
>
> -8.1 37.5
>
> But my trouble is that I want to fish the CRS from the ogr2ogr dataset 
> (the ds2 above) so that I can convert back from GDAL to GMT (which I 
> already can) without having to save to a file in disk.
>
> So basically my question is where is that CRS stored and how can I 
> access it?
>
> Thanks
>
> Joaquim
>
>
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