[GIS] Сalculate polygon centroids with ‘point on surface’ function

centroidsminidumpqgis

I want to calculate centroid coordinates X_center and Y_center for every polygon in layer so that in case polygon has not simple shape (e.g. with holes or like in the picture below) centroid will be in the polygon anyway:

I found 'point on surface' function in QGIS, that returns a point guaranteed to lie on the surface of a geometry. BUT When trying to calculate coordinates with the function, dump happens, although qgis shows output preview correctly –

I found similar question but the following answer implicates new point layer creation, that is useless for me – I want to get centroid coordinates in the same layer.

Interesting, when creating virtual field – it calculates, I can even pick the polygon and see coordinates, but when opening attribute table, qgis crushes.

What is wrong?

Best Answer

Sounds like something in your geometry is causing the issue. I'd check your geometry validity, and also open a bug report for qgis and attach your original dataset.

Related Solutions

[GIS] Polygon Centroid Options and Appropriateness (FeatureToPoint, Labels, etc)

For a general clarification I would say that a centroid may lie outside the polygon, but with the within option it is selecting a point within the polygon but may not be a true centroid, so in both instances the point would be in different locations.

[GIS] assign and move centroids to their polygons

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()

Best Answer

Related Solutions

[GIS] Polygon Centroid Options and Appropriateness (FeatureToPoint, Labels, etc)

[GIS] assign and move centroids to their polygons

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