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Fix incorrect area calculation in corner cases (fix #16820)
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In certain circumstances very proximal nodes could cause instability
in the ellipsoidal area calculation.

Port (slightly tweaked) fix from grass changeset 71167 for same issue,
and add a unit test

(cherry-picked from d850393)
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nyalldawson committed Jul 10, 2017
1 parent fda97b2 commit a0d6412
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Showing 2 changed files with 40 additions and 4 deletions.
33 changes: 29 additions & 4 deletions src/core/qgsdistancearea.cpp
Original file line number Diff line number Diff line change
Expand Up @@ -897,6 +897,14 @@ double QgsDistanceArea::computePolygonArea( const QList<QgsPoint>& points ) cons
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.
*/
const double thresh = 0.7e-7;

QgsDebugMsgLevel( "Ellipsoid: " + mEllipsoid, 3 );
if (( ! mEllipsoidalMode ) || ( mEllipsoid == GEO_NONE ) )
{
Expand Down Expand Up @@ -927,11 +935,28 @@ double QgsDistanceArea::computePolygonArea( const QList<QgsPoint>& points ) cons
x1 += m_TwoPI;

dx = x2 - x1;
area += dx * ( m_Qp - getQ( y2 ) );

dy = y2 - y1;
if ( !qgsDoubleNear( dy, 0.0 ) )
area += dx * getQ( y2 ) - ( dx / dy ) * ( Qbar2 - Qbar1 );
if ( qAbs( dy ) > thresh )
{
/* account for different latitudes y1, y2 */
area += dx * ( m_Qp - ( Qbar2 - Qbar1 ) / dy );
}
else
{
/* latitudes y1, y2 are (nearly) identical */

/* if y2 becomes similar to y1, i.e. y2 -> y1
* Qbar2 - Qbar1 -> 0 and dy -> 0
* (Qbar2 - Qbar1) / dy -> ?
* (Qbar2 - Qbar1) / dy should approach Q((y1 + y2) / 2)
* Metz 2017
*/
area += dx * ( m_Qp - getQ( y2 ) );

/* original:
* area += dx * getQ( y2 ) - ( dx / dy ) * ( Qbar2 - Qbar1 );
*/
}
}
if (( area *= m_AE ) < 0.0 )
area = -area;
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11 changes: 11 additions & 0 deletions tests/src/core/testqgsdistancearea.cpp
Original file line number Diff line number Diff line change
Expand Up @@ -45,6 +45,7 @@ class TestQgsDistanceArea: public QObject
void measureAreaAndUnits();
void emptyPolygon();
void regression14675();
void regression16820();

};

Expand Down Expand Up @@ -370,6 +371,16 @@ void TestQgsDistanceArea::regression14675()
QGSCOMPARENEAR( calc.measureArea( &geom ), 0.83301, 0.02 );
}

void TestQgsDistanceArea::regression16820()
{
QgsDistanceArea calc;
calc.setEllipsoid( "WGS84" );
calc.setSourceCrs( QgsCoordinateReferenceSystem( "EPSG:32634" ) );
QgsGeometry geom( QgsGeometryFactory::geomFromWkt( "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, 0.2 );
}

QTEST_MAIN( TestQgsDistanceArea )
#include "testqgsdistancearea.moc"

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