[Gdal-dev] OGR Questions
warmerdam at pobox.com
Sat Dec 13 01:55:35 EST 2003
Curt Mills wrote:
> 1) Can the coordinate system change per layer in a vector file? If
> not, shouldn't the call to get the spatial reference be based on the
> datasource instead of the layer?
In theory different layers in a datasource can have different coordinate
systems, though this is uncommon. It does happen though.
> 2) Shapefile "hole" polygons: Are there other OGR formats that do
> something similar? Should I check whether the datasource is a
> shapefile and implement "hole" polygons only for them?
I'm not sure what a hole polygon is. A shapefile polygon file may
have Polygon and MultiPolygon geometries. Till just recently multipolygons
were just returned as polygons, but Radim recently fixed this.
What do you mean?
> 3) Is there a standard anywhere for how to specify drawing
> preferences across GIS systems? I'm talking about things like
> enabling/disabling layers, deciding which field is the label for the
> object, colors, layer stackup, line widths, polygon fill or outline,
> etc. The sorts of things that might be written to a file that
> accompanies a dataset, or perhaps covers a directory heirarchy of
> GIS files.
There are many standards. Within the OGR API the preferred approach
to this is the OGR Feature Style Specification:
Basically this is intended to provide a mechanism for different underlying
formats to indicate their rendering information. It is at least partially
implemented for the DGN and Mapinfo drivers. The only applications that I
am aware of taking advantage of it are OpenEV and MapServer.
> 4) Getting the number of points in a feature: Is that always an
> "inexpensive" operation? If it can be "expensive", is there a way
> to gain the entire set of vectors instead that would be faster? I
> saw nothing in ogr_api.h about it.
> Again, I'm using the C API to OGR.
Getting the number of points in an OGRGeometry is generally cheap. If the
geometry is a collection,then you would have to dig through the tree to get
the total count, but it is still relatively cheap.
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