[R-sig-Geo] spherical polygon

Dylan Beaudette dylan.beaudette at gmail.com
Mon Feb 8 02:30:23 CET 2010

How about an equal-area map projection?



On Sun, Feb 7, 2010 at 1:37 PM, Matthew Vavrek
<matthew at matthewvavrek.com> wrote:
> Hello,
> I have a question that I have yet to find a definitive answer for on this
> mailing list (or elsewhere as of yet) about a method of calculating the area
> of a spherical polygon (ie an area on the surface of the earth). I know of
> the areapl() function in the splancs package, although as I understand it
> can only calculate the area of a planar polygon, not a spherical one. The
> polygons I'm trying to calculate are generally simple and convex; is there
> any way of doing this in R?
> Also, I would like to get some feedback on a function that I wrote myself
> that does this (although not perfectly). The function is called earth.poly()
> and can be found in the fossil package on CRAN. Right now, it can only deal
> with areas less than one hemisphere in size, and I haven't stress tested it
> too much yet. Also, it treats the earth as a sphere rather than a true
> ellipsoid for simplicity, which can mean up to a 0.2% error in actual area
> (although it is generally much less). Basically though, if anyone knows of
> something better out there already, I would gladly use that instead, rather
> than spending far too much time doing it myself.
> Cheers
> Matthew
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