topic Re: Some errors on Intersection by ITopologicalOperator in ArcObjects SDK Questions

Some errors on Intersection by ITopologicalOperator

MerhutOqaz — Thu, 24 Jun 2010 22:32:33 GMT

I'm trying to intersect some polygons (shapes from graphic objects, not features) using the Intersect method of the ITopologicalOperator class. On the same geometries sometime it works and sometime not.
The code I use is

        Dim pPoly1, pPoly2, pIntersection As IPolygon
        ..
        .. 'give a square shape to the polygons
        ..
        Dim opTop As ITopologicalOperator3 = pPoly1
        pIntersection = opTop.Intersect(pPoly2, esriGeometryDimension.esriGeometry2Dimension)

I've tried also with ITopologicalOperator and ITopologicalOperator2 with the same results.
I can not find a pattern for the uncorrect behaviour except that it seems to get worst the more I move one polygon far right to the other (I know it's strange). In the attached picture the green area is pIntersection .
Any idea?
Thank you in advance
Valentino

Re: Some errors on Intersection by ITopologicalOperator

GeorgeSilva — Wed, 30 Jun 2010 18:10:02 GMT

Hello valentino,

ITopologicalOperator needs to have accurate ISpatialReference values, otherwise it may give bizzare results.

Are you declaring the ISpatialReference for your graphics? You features might have Spatial Refrences, but what about the graphics?

Re: Some errors on Intersection by ITopologicalOperator

Anonymous User — Wed, 30 Jun 2010 21:36:37 GMT

Original User: kirkktx

Also, don't assume the geometry of an IElement is Simple - try simplifying be setting IsKnownSimple to false and call Simplify. In addition to George's spatialref advice, consider calling IGeometry.SnapToSpatialReference.

Re: Some errors on Intersection by ITopologicalOperator

MerhutOqaz — Thu, 29 Jul 2010 11:44:35 GMT

Thank you, George and Kirk.
Following your advices I've added the lines of code below for all the polygons involved but it still doesn't work!

pPoly1.SpatialReference = m_spatialReference      'spatial reference of the map
pPoly1.SnapToSpatialReference()
Dim opTop As ITopologicalOperator4 = pPoly1
If (Not opTop.IsknowSimple) Then                       'isKnowSimple is readonly, I can not set to False
       opTop.Simplify()
End If
'**** make intersect or any other opTop method

I really don't know what to do. That operator is involved in almost any geometrical operation I need to do on map objects.

Re: Some errors on Intersection by ITopologicalOperator

Anonymous User — Thu, 29 Jul 2010 13:26:54 GMT

Original User: kirkktx

try opTop.IsKnownSimple_2 = false