[R-sig-Geo] computing square kilometers in shape file polygons

[brwin338]

Good Afternoon
Does any one know of a package/function that could be used to compute the 
square kilometers for polygons in a long-lat projected shapefile?
I can access the values in the "area" slots in the object resulting from using 
maptool's  "readShapePoly" command.  I have approximated the km^2 from these 
values by using one of the polygons's (such as a given state's) known area in 
km^2 and rescaling the polygon's "area" relative to the state's known area.  I 
then applied this "scaling factor" to the other polygons in my shapefile object 
to approximate the other polygons' are in km^2.

However, if I understand things correctly, my approximate "area to km^2" 
scaling  will become increasing distorted as I move north or south relative to 
my original "normalizing" state.  Is there a function that can compensate for 
changes in latitude when computing the polygon areas associated with long-lat 
polygons?  I hope this question is somewhat coherent
Thanks
Joe     




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