[R-sig-Geo] how to calculate areas of shape files using R
Robert Hijmans
r.hijmans at gmail.com
Tue Feb 1 08:58:11 CET 2011
> A small caveat - if the coordinates are planar, the explanation holds, if
> geographical (degrees), it doesn't. The naive area values have been used
> internally to plot larger Polygon objects before smaller Polygon objects,
> to avoid overplotting in earlier versions of R that did not "identify"
> holes automatically. So first make sure that you have planar coordinates,
> otherwise the "area" values have no metric. If need be, project with
> spTransform() methods in rgdal
If the coordinates are spherical (in degrees) you can compute the area of
polygons in a SpatialPolygons* object like this:
library(geosphere)
areaPolygons(object)
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