[Gdal-dev] How to Create a Geometry Filter

[Frank Warmerdam]

Kent Eschenberg wrote:
> Greetings,
> 
> This is a combination complaint/question. I'm trying to use the layer 
> classes'  geometry filter to select only features in an area bounded by 
> 4 points.
> 
> I can't do that directly but must use SetSpatialFilter and first create 
> an instance of OGRPolygon. I do so and look for a way to set the 
> polygon's points.
> 
> I can't do that directly but must use addRing and first create an 
> instance of OGRLinearRing. I do that and look for a way to set the 
> ring's points.
> 
> I can't do that directly but must use importFromWkb and first create a 
> buffer of "the well known binary data".

Kent,

The convenience function SetSpatialFilterRect() shows the simpliest
approach (in C++) to create a filter polygon with four points.

void OGRLayer::SetSpatialFilterRect( double dfMinX, double dfMinY,
                                      double dfMaxX, double dfMaxY )

{
     OGRLinearRing  oRing;
     OGRPolygon oPoly;

     oRing.addPoint( dfMinX, dfMinY );
     oRing.addPoint( dfMinX, dfMaxY );
     oRing.addPoint( dfMaxX, dfMaxY );
     oRing.addPoint( dfMaxX, dfMinY );
     oRing.addPoint( dfMinX, dfMinY );

     oPoly.addRing( &oRing );

     SetSpatialFilter( &oPoly );
}

In short, it is not normal practice to create WKB in programs, since
that is ... not very convenient.  It is a useful interchange format
with other systems (ie. postgis).

The OGRLinearRing isn't a "proper" OGC simple features geometry and
so has no corresponding it WKB format.  OGRLinearRings are only
allowed to exist in the context of a polygon.

I hope this helps.

Best regards,
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