How to use the IRelationalOperator interface to check if a point feature within a polygon feature?

Question

I use the following code to check if a point feature is within a polygon feature

pFeaBlock is a polygon feature and Fea is a point feature

int blockID = -1;
IFeatureCursor pBlockCursor = this._blockLayer.Search(null, false);
IFeature pFeaBlock = pBlockCursor.NextFeature();
while (pFeaBlock != null)
{
    if (!pFea.Shape.IsEmpty)
    {
      IPolygon pPolygon = (IPolygon)pFea.Shape;
      IRelationalOperator pRelOperator = (IRelationalOperator)pPolygon;
      if (pRelOperator.Within(pFeaBlock.Shape) == true)
      {
          blockID = (int)pFeaBlock.get_Value(2);
      }
     }

     pFeaBlock = pBlockCursor.NextFeature();
 }

but I always get he COMException on

if (pRelOperator.Within(pFeaBlock.Shape) == true)

Is anyone familiar this interface?

Answer 1

First, check if the relational Operator is Null. But your problem is with the geometries: You need to be sure that the relational operatior can be used. For that both spatial references need to be the same. You can useSpatialReference.FactoryCodefor check it.

while ((IFeature pFeaBlock = pBlockCursor.NextFeature()) != null)
{
  if (!pFea.Shape.IsEmpty)
  {
    IPolygon pPolygon = (IPolygon)pFea.Shape;
    if (pPolygon.SpatialReference.FactoryCode == pFeaBlock.Shape.SpatialReference.FactoryCode)
    {        
      IRelationalOperator pRelOperator = (IRelationalOperator)pPolygon;
      if (pRelOperator.Within(pFeaBlock.Shape) == true)
      {
        blockID = (int)pFeaBlock.get_Value(2);
      }
    }
  }   
}

Answer 2

for checking if polygon contains a point it's better to use the Contain method instead the Within method.

Dim aRing As IPointCollection = New RingClass()
aRing.AddPoint(new Point() With {.X=412853 ,.Y = 3785339})
aRing.AddPoint(new Point() With {.X=412990 ,.Y = 3785276})
aRing.AddPoint(new Point() With {.X=412926 ,.Y = 3784889})
aRing.AddPoint(new Point() With {.X=412782 ,.Y = 3784940})
aRing.AddPoint(new Point() With {.X=412853 ,.Y = 3785339})
Dim geo As IGeometry = aRing
Dim poly As IGeometryCollection = New Polygon()
poly.AddGeometry(aRing)
Dim spdef As New SpatialReferenceEnvironment()

Dim op As IRelationalOperator = poly
Dim poly1 As IPolygon = poly
poly1.SpatialReference = spdef.CreateProjectedCoordinateSystem(esriSRProjCSType.esriSRProjCS_WGS1972UTM_39N)

Dim outpoint As IPoint = New Point() With {.X = 412728, .Y = 3785165}
outpoint.SpatialReference = spdef.CreateProjectedCoordinateSystem(esriSRProjCSType.esriSRProjCS_WGS1972UTM_39N)

Dim insidePoint As IPoint = New Point() With {.X = 412878, .Y = 3785119}
insidePoint.SpatialReference = spdef.CreateProjectedCoordinateSystem(esriSRProjCSType.esriSRProjCS_WGS1972UTM_39N)

Dim results = op.Contains(outpoint)' return false
Dim results = op.Contains(insidePoint) ' return true