Change full circle interpretation, fixes #15116

Author: mhugent

src/core/geometry/qgscircularstringv2.cpp

@@ -879,7 +879,7 @@ void QgsCircularStringV2::sumUpArea( double& sum ) const
     //segment is a full circle, p2 is the center point
     if ( p1 == p3 )
     {
-      double r2 = QgsGeometryUtils::sqrDistance2D( p1, p2 );
+      double r2 = QgsGeometryUtils::sqrDistance2D( p1, p2 ) / 2.0;
       sum += M_PI * r2;
       continue;
     }

src/core/geometry/qgsgeometryutils.cpp

@@ -359,9 +359,9 @@ void QgsGeometryUtils::circleCenterRadius( const QgsPointV2& pt1, const QgsPoint
   //closed circle
   if ( qgsDoubleNear( pt1.x(), pt3.x() ) && qgsDoubleNear( pt1.y(), pt3.y() ) )
   {
-    centerX = pt2.x();
-    centerY = pt2.y();
-    radius = sqrt( pow( pt2.x() - pt1.x(), 2.0 ) + pow( pt2.y() - pt1.y(), 2.0 ) );
+    centerX = ( pt1.x() + pt2.x() ) / 2.0;
+    centerY = ( pt1.y() + pt2.y() ) / 2.0;
+    radius = sqrt( pow( centerX - pt1.x(), 2.0 ) + pow( centerY - pt1.y(), 2.0 ) );
     return;
   }