For a python test script I would like to loop through a MultiPolygon,
calculation the areas of the single Polygons, exteriorRing and where they
exist the interiorRing and compare the results from another source
(spatialite).
Sample code:
test_polygon = test_geom.geometryN(index)
self.assertEqual(test_polygon.wkbType(), QgsWKBTypes.Polygon)
count_rings=test_polygon.numInteriorRings()
if (count_rings > 0):
polygon_ring_area_sum+=polygon_area
exterior_ring = test_polygon.exteriorRing()
print('-I-> Polygon[%d] : exteriorRing[%s] isClosed[%d] isRing[%d] '%
(index,
exterior_ring.geometryType(),exterior_ring.isClosed(),exterior_ring.isRing()))
i_rc=exterior_ring.convertTo(QgsWKBTypes.Polygon)
exterior_ring_area=round(exterior_ring.area(),6)
-I-> Polygon[11] : exteriorRing[LineString] isClosed[1] isRing[1]
-I-> Polygon[11] : exteriorRing[LineString] exterior_ring_area[0.000000]
rc=0
To calculate the area, the geometry must be a Polygon but I have not found
a way to do this properly.
The convertTo() method seems to only change to a Z or M type
- the documentation is a bit vague 'Converts the geometry to a specified
type.'
the next attempt:
line = QgsGeometry.fromWkt(exterior_ring.asWkt())
exterior_ring_polygon = line.convertToType(QGis.Polygon, False)
also did not bring the desired result (fails when creating 'line').
Since the returned LineString is closed and a ring, there should be no
problem.
--
A second question:
How can the srid of a geometry retrieved from a QgsVectorLayer be
determined?
Mark Johnson
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