Replace the hardcoded treshold in line splitting code and added more …

…comment

git-svn-id: http://svn.osgeo.org/qgis/trunk/qgis@10935 c8812cc2-4d05-0410-92ff-de0c093fc19c

src/core/qgsgeometry.cpp

@@ -4794,18 +4794,27 @@ int QgsGeometry::splitLinearGeometry( GEOSGeometry *splitLine, QList<QgsGeometry
   QVector<GEOSGeometry*> testedGeometries;
   GEOSGeometry* intersectGeom = 0;
 
-  //hardcoded thresholds
+  //Create a small buffer around the original geometry
+  //and intersect candidate line segments with the buffer.
+  //Then we use the ratio intersection length / segment length to
+  //decide if the line segment belongs to the original geometry or
+  //if it is part of the splitting line
   double bufferDistance = 0.0000001;
-  double intersectThreshold =  0.00001;
 
   for ( int i = 0; i < GEOSGetNumGeometries( mergedLines ); i++ )
   {
     const GEOSGeometry *testing = GEOSGetGeometryN( mergedLines, i );
     intersectGeom = GEOSIntersection( mGeos, GEOSBuffer( testing, bufferDistance, DEFAULT_QUADRANT_SEGMENTS ) );
     double len;
     GEOSLength( intersectGeom, &len );
-    if ( len > intersectThreshold )
+     double testingLen;
+    GEOSLength( testing, &testingLen);
+    double ratio = len / testingLen;
+    //the ratios for geometries that belong to the original line are usually close to 1
+    if ( ratio >= 0.5 && ratio <= 1.5)
+    {
       testedGeometries << GEOSGeom_clone( testing );
+    }
     GEOSGeom_destroy( intersectGeom );
   }