Area of projected map regions
Computes the areas of regions in a projected map.
area.map(m, regions = ".", sqmi=TRUE, ...)
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.
The correct use of this function is to first use map to
create polygons and project the coordinates onto a plane, then apply
area.map to compute the area of the projected regions.
If the projection is area-preserving (such as albers),
then these areas will match the area on the globe, up to a
constant. To get an absolute area in square miles, the sqmi
option will scale the result, depending on the projection.
The coordinates from map are affected by
its resolution argument, so use resolution=0 for the most
accurate areas.
a named vector of region areas.
The sqmi option assumes the coordinates have been projected
with the mapproject function.
Tom Minka
area.polygon, apply.polygon
# because the projection is rectangular, these are not true areas on the globe.
m = map("state", fill = TRUE, plot = FALSE)
area.map(m)
area.map(m, ".*dakota")
area.map(m, c("North Dakota", "South Dakota"))
if(require(mapproj)) {
# true areas on the globe
m = map("state", proj="bonne", param=45, fill=TRUE, plot=FALSE)
# North Dakota is listed as 70,704 square miles
area.map(m, "North Dakota")
}Please choose more modern alternatives, such as Google Chrome or Mozilla Firefox.