# [postgis-users] How is Centroid(Geometry) calculated?

Obe, Regina robe.dnd at cityofboston.gov
Thu Jun 14 04:52:03 PDT 2007

```It uses the Geos library to calculate centroid if Geos is installed.  My
cursory glance of the Geos code,  looks to me like for polygons and
multipolygons
it first takes the area of the region and then finds the average x,y, z
of that encompassing area.

So yes it is conceivable in a multipolygon that the centroid may not
fall in any of the polygons.

Anyone correct me if I am wrong since I'm not too familiar with c++
code.

Anyrate I think the relevant piece of code in geos library is
source\algorithm\CentroidArea.cpp

Hope that helps,
Regina

________________________________

From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Matt
Doughty
Sent: Thursday, June 14, 2007 7:03 AM
To: postgis-users at postgis.refractions.net
Subject: [postgis-users] How is Centroid(Geometry) calculated?

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 Doughty

GEOGRAMA S.L.

Tel.:  +34 945 13 13 72    652 77 14 15

Fax: +34 945 23 03 40

www.geograma.com

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