On 14 juin 07, at 13:02, Matt Doughty wrote:
Hi,
Can anybody explain how PostGIS calculates centroids? I’ve checked
the manual but the relevant section doesn’t refer to an OGC
Specification. From what I can understand, it takes an average of
the coordinates of all the vertices of an entry, ignoring whether
or not the centroid falls outside the polygons. Is this right? This
differs from other GIS- GeoMedia, for example, when confronted with
a multipolygon entry, always puts the centroid inside the largest
polygon.
Can anybody shed some light on this?
Cheers,
Matt
Matt,
I don't know the algorithm, but you're right, centroid is not
necessary inside polygon if it is convex, with interior rings
(centroid inside a interior ring) or a (i did not check myself) multi-
polygon. As far as i can see, the centroid is a straight forward
barycenter (mean of all points), and therefore you cannot really use
it as a "center point" in a user interface (to place a GUI element
for a polygon that is not inside polygon confuses the user).
I agree, having a function that would give a kind of centroid
guaranteed to be inside the polygon, and in a "natural way" (eg
equidistant from edges) would be very nice.
Regards
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