[R-sig-Geo] how to calculate areas of shape files using R

[Robert Hijmans]

> 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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