用ITopologicalOperator.union进行多边形合并时，需要注意
从www.esri.com里面找到的资料，贴出来：
When working with the ITopologicalOperator interface, there are 3 things that should be done.
1) QI over to ITopologicalOperator2 and set IsKnownSimple to False, then call Simplify. This should be done for ALL geometries involved.
2) ALL geometries involved should have a spatial reference set. If they don't, then set each geometry's spatial reference to that of the map or something (just set it). If ALL geometries involved do not have the SAME spatial reference, then project ALL geometries into the SAME spatial reference (doesn't matter which you use, just use one of them).
3) Call SnapToSpatialReference on all geometries involved.
After doing this, you will greatly increase your chances of making it through the operation without error. It does not work ALL of the time. ITopologicalOperator is one buggy interface, but doing the 3 things above every time you use it will cut down the number of errors you get significantly. Good luck.
Here's some sample VB code that shows one way to do all of this.
Set pTopoOp = pPolygon1
pTopoOp.IsKnownSimple = False
pTopoOp.Simplify
Set pTopoOp = pPolygon2
pTopoOp.IsKnownSimple = False
pTopoOp.Simplify
If pPolygon1.SpatialReference Is Nothing Then
Set pPolygon1.SpatialReference = pMap.SpatialReference
End If
If pPolygon2.SpatialReference Is Nothing Then
Set pPolygon2.SpatialReference = pMap.SpatialReference
End If
pPolygon2.Project pPolygon1.SpatialReference
pPolygon1.SnapToSpatialReference
pPolygon2.SnapToSpatialReference
' now you can FINALLY do the Union.
Set pPolygon1 = pTopoOp.Union(pPolygon1)
' hopefully you get to here.
MsgBox "Done"
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