[R-sig-Geo] spherical polygon

[Matthew Vavrek]

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