Polyline intersect on polygon edge

Question

I have a polyline that passes through a polygon, I???m trying to get the two intersect points on the polygon edge. If I try to get the fist and last of what is returned for the intersect point collection I get inconsistent results.

Does anyone have some input as what I might be doing wrong?  I have my code block below.

Appreciate the help
vb
  
  
  Polyline outGeometry = new Polyline();

IGeometryCollection geometryCollection = null;
  if(geometryCollection == null) {
   IGeometryBag geometryBag = new GeometryBag();
   geometryCollection = (IGeometryCollection) geometryBag;
   geometryBag.setSpatialReferenceByRef(mySpatialRef));
  }

// union all your polylines
  for(int i=0; i<featureList.size(); i++) {
   geometryCollection.addGeometry(featureList.get(i).getShape(), null, null);
  }
  outGeometry.constructUnion((IEnumGeometry) geometryCollection);   
  ((ITopologicalOperator) outGeometry).simplify(); 
  
  ITopologicalOperator topo ;
  geometry = ((ITopologicalOperator)geometry).getBoundary();
  topo = (ITopologicalOperator)geometry  ; 
  
  
IGeometry intersects = topo.intersect((IGeometry) outGeometry,    esriGeometryDimension.esriGeometry0Dimension);
  
  IPointCollection Pts = (IPointCollection)intersects;
  for (int j=0;j<Pts.getPointCount();j++){
   tempIntersectPointList.addPoint(Pts.getPoint(j), null, null);
  }

firstPointOnEdge = tempIntersectPointList.getPoint(0)
secondPointOnEdge = tempIntersectPointList.getPoint(tempIntersectPointList.getPointCount() - 1),

kenbuja · Answer

Try the IPolygon4::ExteriorRingBag
Dim pPolygon As ESRI.ArcGIS.Geometry.IPolygon4 = pPolygonGeometry
Dim pGeoBag As ESRI.ArcGIS.Geometry.IGeometryBag = pPolygon.ExteriorRingBag
Dim pGeoCollection As ESRI.ArcGIS.Geometry.IGeometryCollection = pGeoBag

MsgBox(pGeoCollection.GeometryCount)

DanielLaidlaw · Answer

Its all comming together , yes I have a polygon with no internal holes. I though the getBoundary() returns a polygon with the donut hole...  So my question would be how do I get just the external ring of the polygon?

vb

kenbuja · Answer

I seem to get the intersecting points fine, my only issue is I cant seem  to identify the point on the polygon edge. Getting the first and last  point on the intersect point collection is not always the point on the  edge of the polygon.  That's what I'm still tryingto figure out.

It looks like it returns the points from the top to the bottom. I ran a test using the code below on a polygon and line, flashing each point in the geometry collection. The image below shows the direction the line was digitized by the arrows. The upper right intersection point was flashed first.Another question if someone knows the answer is why do I get so many  points when I only use the polygon boundary to intersect the polyline.  Shouldn�??t it just be two?

Is it a polygon with no internal holes? In the sample I ran, I used a donut polygon, which resulted in four intersections. If you look at the help for Boundary, you'll see that it will return all the rings of a polygon, not just the outside boundary.
            Dim pLayer As ESRI.ArcGIS.Carto.ILayer2
            Dim pLayer1 As ESRI.ArcGIS.Carto.ILayer2
            Dim pFLayer As ESRI.ArcGIS.Carto.IFeatureLayer
            Dim pFClass As ESRI.ArcGIS.Geodatabase.IFeatureClass
            Dim pFCursor As ESRI.ArcGIS.Geodatabase.IFeatureCursor
            Dim pFeature As ESRI.ArcGIS.Geodatabase.IFeature
            Dim pPolygonGeometry As ESRI.ArcGIS.Geometry.IGeometry
            Dim pLineGeometry As ESRI.ArcGIS.Geometry.IGeometry
            Dim pTopoOp As ESRI.ArcGIS.Geometry.ITopologicalOperator
            Dim pOverlapGeometryCollection As ESRI.ArcGIS.Geometry.IGeometryCollection
            Dim pOverlapGeometry As ESRI.ArcGIS.Geometry.IGeometry
            Dim pRgbColor As New ESRI.ArcGIS.Display.RgbColor

Using releaser As New ESRI.ArcGIS.ADF.ComReleaser
                releaser.ManageLifetime(pFCursor)

pLayer = My.ArcMap.Document.FocusMap.Layer(0)
                pFLayer = New ESRI.ArcGIS.Carto.FeatureLayer
                pFLayer = pLayer
                pFClass = pFLayer.FeatureClass
                pFCursor = pFClass.Search(Nothing, False)
                pFeature = pFCursor.NextFeature
                pLineGeometry = pFeature.Shape

pLayer1 = My.ArcMap.Document.FocusMap.Layer(1)
                pFLayer = New ESRI.ArcGIS.Carto.FeatureLayer
                pFLayer = pLayer1
                pFClass = pFLayer.FeatureClass
                pFCursor = pFClass.Search(Nothing, False)
                pFeature = pFCursor.NextFeature
                pPolygonGeometry = pFeature.Shape

pTopoOp = pPolygonGeometry
                pOverlapGeometryCollection = pTopoOp.Intersect(pLineGeometry, ESRI.ArcGIS.Geometry.esriGeometryDimension.esriGeometry0Dimension)

MsgBox(pOverlapGeometryCollection.GeometryCount)
                pRgbColor.Red = 255

For j As Integer = 0 To pOverlapGeometryCollection.GeometryCount - 1
                    pOverlapGeometry = pOverlapGeometryCollection.Geometry(j)
                    FlashGeometry(pOverlapGeometry, pRgbColor, My.ArcMap.Document.ActiveView.ScreenDisplay, 500) ' this is an ArcGIS snippet
                Next
           End Using

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intersect.jpg