Port grass followup to area calculation fix

https://trac.osgeo.org/grass/changeset/71259

refs discussion at

https://trac.osgeo.org/grass/ticket/3369

Author: nyalldawson

src/core/qgsdistancearea.cpp

@@ -674,17 +674,18 @@ double QgsDistanceArea::computePolygonArea( const QList<QgsPointXY> &points ) co
     return 0;
   }
 
+  // IMPORTANT
+  // don't change anything here without reporting the changes to upstream (GRASS)
+  // let's all be good opensource citizens and share the improvements!
+
   double x1, y1, x2, y2, dx, dy;
   double Qbar1, Qbar2;
   double area;
 
   /* GRASS comment: threshold for dy, should be between 1e-4 and 1e-7
-   * QGIS note: while the grass comment states that thres should be between 1e-4->1e-7,
-   * a value of 1e-7 caused TestQgsDistanceArea::regression14675() to regress
-   * The maximum threshold possible which permits regression14675() to pass
-   * was found to be ~0.7e-7.
+   * See relevant discussion at https://trac.osgeo.org/grass/ticket/3369
   */
-  const double thresh = 0.7e-7;
+  const double thresh = 1e-6;
 
   QgsDebugMsgLevel( "Ellipsoid: " + mEllipsoid, 3 );
   if ( !willUseEllipsoid() )
@@ -732,7 +733,7 @@ double QgsDistanceArea::computePolygonArea( const QList<QgsPointXY> &points ) co
        * (Qbar2 - Qbar1) / dy should approach Q((y1 + y2) / 2)
        * Metz 2017
        */
-      area += dx * ( m_Qp - getQ( y2 ) );
+      area += dx * ( m_Qp - getQ( ( y1 + y2 ) / 2.0 ) );
 
       /* original:
        * area += dx * getQ( y2 ) - ( dx / dy ) * ( Qbar2 - Qbar1 );

tests/src/core/testqgsdistancearea.cpp

@@ -377,7 +377,7 @@ void TestQgsDistanceArea::regression14675()
   calc.setSourceCrs( QgsCoordinateReferenceSystem::fromSrsId( 145L ) );
   QgsGeometry geom( QgsGeometryFactory::geomFromWkt( QStringLiteral( "Polygon ((917593.5791854317067191 6833700.00807378999888897, 917596.43389983859378844 6833700.67099479306489229, 917599.53056440979707986 6833700.78673478215932846, 917593.5791854317067191 6833700.00807378999888897))" ) ) );
   //lots of tolerance here - the formulas get quite unstable with small areas due to division by very small floats
-  QGSCOMPARENEAR( calc.measureArea( geom ), 0.83301, 0.02 );
+  QGSCOMPARENEAR( calc.measureArea( geom ), 0.833010, 0.03 );
 }
 
 void TestQgsDistanceArea::regression16820()
@@ -387,7 +387,7 @@ void TestQgsDistanceArea::regression16820()
   calc.setSourceCrs( QgsCoordinateReferenceSystem( QStringLiteral( "EPSG:32634" ) ) );
   QgsGeometry geom( QgsGeometryFactory::geomFromWkt( QStringLiteral( "Polygon ((110250.54038314701756462 5084495.57398066483438015, 110243.46975068224128336 5084507.17200060561299324, 110251.23908144699817058 5084506.68309532757848501, 110251.2394439501222223 5084506.68307251576334238, 110250.54048078990308568 5084495.57553235255181789, 110250.54038314701756462 5084495.57398066483438015))" ) ) );
   //lots of tolerance here - the formulas get quite unstable with small areas due to division by very small floats
-  QGSCOMPARENEAR( calc.measureArea( geom ), 43.183369, 0.02 );
+  QGSCOMPARENEAR( calc.measureArea( geom ), 43.3280029296875, 0.2 );
 }
 
 QGSTEST_MAIN( TestQgsDistanceArea )