topic Re: ITopologicalOperator.Difference in Developers Questions

ITopologicalOperator.Difference

HagenProbsthain1 — Wed, 29 Nov 2017 09:17:32 GMT

Does anyone have some experience with ITopologicalOperator:Difference?

I want to erase features or feature parts (points, multipoints, lines or polygons) inside the polygons of a polygon featureclass.

I'm looking for an example how to use it. I always get an error when calling the method:

I know, in the helptext is written: "This method does not support GeometryBags."

But it works for ConstructUnion in the same manner, so I think that's not the problem.

(infeatclass is type of multipoint, erasefeatclass is type of polygon)

Friend Function funcEraseTest(pInFClass As IFeatureClass, _pEraseFClass As IFeatureClass) As IGeometry

' put all geometries to be erased into a collection

Dim pPointSearchCur As IFeatureCursor = pInFClass.Search(Nothing, False)

Dim pPointGeomBag As IGeometryBag = New GeometryBag

Dim pPointGeomColl As IGeometryCollection = pPointGeomBag

Dim pSimplify As ITopologicalOperator2

Dim pFeat As IFeature

pFeat = pPointSearchCur.NextFeature

Do While Not pFeat Is Nothing

pSimplify = pFeat.ShapeCopy

pSimplify.IsKnownSimple_2 = False

pSimplify.Simplify()

pPointGeomColl.AddGeometry(pSimplify)

pFeat = pPointSearchCur.NextFeature

Loop

' put all erase geometries (polys) into a collection

Dim pPolySearchCur = pEraseFClass.Search(Nothing, False)

Dim pEraseGeomBag As IGeometryBag = New GeometryBag

Dim pEraseGeomColl As IGeometryCollection = pEraseGeomBag

Dim pEraseSimplify As ITopologicalOperator2

pFeat = pPolySearchCur.NextFeature

Do While Not pFeat Is Nothing

pEraseSimplify = pFeat.ShapeCopy

pEraseSimplify.IsKnownSimple_2 = False

pEraseSimplify.Simplify()

pEraseGeomColl.AddGeometry(pEraseSimplify)

pFeat = pPolySearchCur.NextFeature

Loop

' make erase

Dim pResultMultiPoint As IMultipoint

Dim pTopoOp As ITopologicalOperator

pTopoOp = pPointGeomColl

' THIS THROWS THE ERROR:

pResultMultiPoint = pTopoOp.Difference(pEraseGeomColl)

Return pResultMultiPoint

End Function

Re: ITopologicalOperator.Difference

AbhishekKabra — Sat, 02 Dec 2017 00:13:36 GMT

Hi,

GeometryBag does not implement all the methods from ITopologicalOperator interface. Buffer, Clip, and Simplify are the only methods of ITopologicalOperator supported on GeometryBags.

I think you can do it like this:

For each multipoint in multipoint_feature_class

For each polygon in polygon_feature_class

      // C++ syntax
      IGeometryPtr ipRes;
      ITopologicalOperatorPtr(multipoint )->Difference(polygon, &ipRes);

Add ipRes to result geometry bag

end

Re: ITopologicalOperator.Difference

HagenProbsthain1 — Mon, 04 Dec 2017 07:48:03 GMT

OK, thank you for your response, I'll try to test it. In the meantime I've called the Toolbox-Tool "Erase" inside my code. It is easier to use as to call the method in loops.

Re: ITopologicalOperator.Difference

HagenProbsthain1 — Mon, 11 Dec 2017 15:10:48 GMT

The problem is, that the documentation says for most of the methods: "This method does not support GeometryBags". But this is not correct, i. e. ConstructUnion works with it. So I thought, the sentence is obsolete and it was forgotten to delete. But at least for the Difference method the documentation is correct.

Therefore my workaround is to use the ConstructUnion method to create a multipart geometry for both input and erase features and then put these geometries to the Difference method. It works fine.