QgsConstWkbPtr QgsDistanceArea::measurePolygon( QgsConstWkbPtr wkbPtr, double* area, double* perimeter, bool hasZptr ) const
{
if ( !wkbPtr )
{
QgsDebugMsg( "no feature to measure" );
return wkbPtr;
}
wkbPtr.readHeader();
// get number of rings in the polygon
int numRings;
wkbPtr >> numRings;
if ( numRings == 0 )
{
QgsDebugMsg( "no rings to measure" );
return QgsConstWkbPtr( nullptr, 0 );
}
// Set pointer to the first ring
QList<QgsPoint> points;
QgsPoint pnt;
double x, y;
if ( area )
*area = 0;
if ( perimeter )
*perimeter = 0;
try
{
for ( int idx = 0; idx < numRings; idx++ )
{
int nPoints;
wkbPtr >> nPoints;
// Extract the points from the WKB and store in a pair of
// vectors.
for ( int jdx = 0; jdx < nPoints; jdx++ )
{
wkbPtr >> x >> y;
if ( hasZptr )
{
// totally ignore Z value
wkbPtr += sizeof( double );
}
pnt = QgsPoint( x, y );
if ( mEllipsoidalMode && ( mEllipsoid != GEO_NONE ) )
{
pnt = mCoordTransform.transform( pnt );
}
points.append( pnt );
}
if ( points.size() > 2 )
{
if ( area )
{
double areaTmp = computePolygonArea( points );
if ( idx == 0 )
{
// exterior ring
*area += areaTmp;
}
else
{
*area -= areaTmp; // interior rings
}
}
if ( perimeter )
{
if ( idx == 0 )
{
// exterior ring
*perimeter += computeDistance( points );
}
}
}
points.clear();
}
}
catch ( QgsCsException &cse )
{
Q_UNUSED( cse );
QgsMessageLog::logMessage( QObject::tr( "Caught a coordinate system exception while trying to transform a point. Unable to calculate polygon area or perimeter." ) );
}
return wkbPtr;
}
