[gdal-dev] GetGeomType() for a MapInfo TAB using OGR MapInfo File driver

[Even Rouault]

Le vendredi 25 avril 2014 18:23:23, Max Demars a écrit :
> Hi Even,
> 
> Thank you for your reply, that explains a lot. I still wonder why there is
> not a possibility to return wkbPolygon like below:
> 
>            if( numPoints > 0 && numLines == 0 && numRegions == 0 )
>                m_poDefn->SetGeomType( wkbPoint );
>            else if( numPoints == 0 && numLines > 0 && numRegions == 0 )
>                m_poDefn->SetGeomType( wkbLineString );
> +++     else if ( numPoints == 0 && numLines == 0 && numRegions > 0 )
> +++         m_poDefn->SetGeomType( wkbPolygon );
>            else
>                /* we leave it unknown indicating a mixture */;
> 
> Maybe there is also a way to check if the layer is made of multi geometries
> of one type (no mixture)... It looks to me that the mitab_
> tabfile.cpp module could be improved that way. What do you think?

I imagine (I haven't coded that driver) that the reason is that regions can be 
either a mix of Polygon and MultiPolygon, hence it is not possible to expose a 
single geometry type.
Well, in the shapefile driver, the same situation occurs and we expose 
wkbPolygon, but can indeed cause some issues when converting to other formats.
A potential solution would be to declare MultiPolygon as geometry type and 
indeed return single Polygon in a MultiPolygon.

> 
> -Max Demars
> 
> 
> On Thu, Apr 24, 2014 at 2:23 PM, Even Rouault
> 
> <even.rouault at mines-paris.org>wrote:
> > Le jeudi 24 avril 2014 19:47:36, Max Demars a écrit :
> > > Hi!
> > > 
> > > I am using the OGR MapInfo File driver to read a MapInfo TAB file.
> > 
> > However,
> > 
> > > the function
> > > GetGeomType()<
> > 
> > http://gdal.org/python/osgeo.ogr.Layer-class.html#GetGeomTyp
> > 
> > > e>returns 0 which means point
> > > geometry, even if the features in the TAB are only multi-polygons.
> > 
> > No, see ogr_core.h :
> >     wkbUnknown = 0,         /**< unknown type, non-standard */
> > > 
> > > Is it because MapInfo TAB can store many different GeomTypes in the
> > > same file?
> > 
> > The MapInfo TAB file will return wkbPoint, wkbLineString or wkbUnknown.
> > The later being for polygons/multipolygons, or mix or geometry types.
> > 
> > Source is ogr/ogrsf_frmts/mitab/mitab_tabfile.cpp :
> >         if( numPoints > 0 && numLines == 0 && numRegions == 0 )
> >         
> >             m_poDefn->SetGeomType( wkbPoint );
> >         
> >         else if( numPoints == 0 && numLines > 0 && numRegions == 0 )
> >         
> >             m_poDefn->SetGeomType( wkbLineString );
> >         
> >         else
> >         
> >             /* we leave it unknown indicating a mixture */;
> > > 
> > > If I take for granted that only one geometry type would always be
> > > founded in the TAB, how could I retrieve it?
> > > 
> > >         driver = ogr.GetDriverByName("MapInfo File")
> > >         datasource = driver.Open(os.path.join(dirname,shapefileName))
> > >         layer = datasource.GetLayer(0)
> > >         geometryType = layer.GetGeomType()
> > >         
> > >         >>>geometryType
> > >         >>>0
> > > 
> > > Even when looping over features in layer and retrieving geometry type
> > 
> > using
> > 
> > > GetDefnRef()<
> > 
> > http://gdal.org/python/osgeo.ogr.Feature-class.html#GetGeometr
> > 
> > > yRef>.GetGeomType() the result is always 0 even for multi-polygon
> > 
> > features.
> > 
> > You should call
> > 
> >         g = feat.GetGeometryRef()
> >         g.GetType()
> > 
> > to retrieve the geometry type of an individual feature, and that cannot
> > be 0=wkbUnknown
> > 
> > layer.GetGeomType(), feat.GetDefnRef().GetGeomType() or
> > layer.GetLayerDefn().GetGeomType() are all alias, that will return the
> > declared layer geometry type, but not the geometry type of an actual
> > feature.
> > 
> > Even
> > 
> > --
> > Geospatial professional services
> > http://even.rouault.free.fr/services.html

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