[GIS] assign and move centroids to their polygons

Question

I m using QGIS 1.8 Lisboa. After calculating the centroids from a polygon shapefile, if some of them lie outside the polygon outlines, I'd like to move them on the polygon they came from. The criterion should be univocal as I must use them to extract coordinates to be used as the identification code of the polygon itself. Therefore, the procedure should output the same point location each time it runs for the same polygon (not a random position like the one given by "random points" function, that gives a new result each times it's ran).

Answer 1

You could use the Shapely python library, which provides a function representative_point() that is guaranteed to lie within the polygon.

Here's a Python script that can be run in the QGIS Python console. The polygon layer for which you want to create the attribute should be selected. The function takes the name of the attribute you want to update. The attribute has to exist in your layer already, it has to be string type, and it should be long enough (30 characters).

Here's an example of the points the algorithm found:

import shapely.wkb

def setIDPoint(attributename):
 layer = qgis.utils.iface.activeLayer()
 provider = layer.dataProvider()
 fields = provider.fields()
 provider.select(provider.attributeIndexes() )
 attributeID = provider.fieldNameIndex(attributename)
 feature = QgsFeature()
 layer.startEditing()
 while provider.nextFeature(feature):
  wkb = feature.geometry().asWkb()
  polygon = shapely.wkb.loads(wkb)
  reprPoint = ','.join([str(polygon.representative_point().x), str(polygon.representative_point().y)] )
  feature.changeAttribute(attributeID, reprPoint)
  layer.updateFeature(feature)
 layer.commitChanges()