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I've spent a little while figuring out the answer to this question. It's not immediately obvious from a Google search, so thought it may useful to post the answer on here. There is also an additional question about non-contiguous polygons.

Instant easy answer: use the command:

centroids <- getSpPPolygonsLabptSlots(polys)

(This was found in the class description of the SpatialPolygonsDataFrame R data class for the overarching spatial package in R, sp)

This seems to do exactly the same thing as

cents <- SpatialPointsDataFrame(coords=cents, data=sids@data, proj4string=CRS("+proj=longlat +ellps=clrk66"))

in the following code, which should be replicable on any R installation (try it!)

sids <- readShapePoly(system.file("shapes/sids.shp", package="maptools")[1], 
                      proj4string=CRS("+proj=longlat +ellps=clrk66"))
writeSpatialShape(sids, "sids")
cents <- coordinates(sids)
cents <- SpatialPointsDataFrame(coords=cents, data=sids@data, 
                  proj4string=CRS("+proj=longlat +ellps=clrk66"))
points(cents, col = "Blue")
writeSpatialShape(cents, "cents")

centroids <- getSpPPolygonsLabptSlots(sids)
points(centroids, pch = 3, col = "Red")

Where cents (blue) and centroids (red) are identical centroids (this should plot should appear after you've run the code):

centroids calculated by R

So far so good. But when you calculate polygon centroids in QGIS (menu: Vector | Geometry | Polygon Centroids ), there are slightly different results for non-contiguous polygons:

QGIS generated polygons

So this question is 3-things:

  1. A quick and easy answer
  2. A warning for people using R to calculate centroids for non-contiguous polygons
  3. A question about how it should be done in R to properly account for multi-part (non-contiguous) polygons
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1 Answer 1

up vote 10 down vote accepted

Firstly, I can't find any documentation that says that coordinates or getSpPPolygonsLabptSlots returns the centre-of-mass centroid. In fact the latter function now shows up as 'Deprecated' and should issue a warning.

What you want for computing the centroid as the centre-of-mass of a feature is the gCentroid function from the rgeos package. Doing help.search("centroid") will have found this.

trueCentroids = gCentroid(sids,byid=TRUE)

should show the difference, and be the same as the Qgis centroids.

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According to Roger Bivand, developer of a number of R's spatial packages, it does: "Yes. The class documentation at ?"Polygons-class" does not state that this is the case, because other points might be validly inserted as label points. The default constructor uses the centroid of the largest non-hole ring in the Polygons object." - Explains non-contiguity. stat.ethz.ch/pipermail/r-help/2009-February/187436.html . Confirmed: gCentroid(sids,byid=TRUE) does indeed solve the problem. –  RobinLovelace Dec 9 '12 at 11:16

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