The area of each matching region in the map is computed, and
regions which match the same element of regions have their
areas combined. Each region is assumed planar, with vertices
specified by the x and y components of the map object.
Note that map returns projected coordinates,
so running area.map on the output of map gives
the area of a region after it is projected onto the screen.
If the projection is area-preserving (such as albers),
then the area on the screen will match the area on the globe, up to a
constant.
The coordinates from map are also affected by
its resolution argument (use resolution=0 for the most
accurate areas).
# because the projection is rectangular, these are not true areas on the globe.m = map("state", fill = TRUE)
area.map(m)
area.map(m, ".*dakota")
area.map(m, c("North Dakota", "South Dakota"))