# area.map: Area of projected map regions In adeckmyn/maps: Draw Geographical Maps

## Description

Computes the areas of regions in a projected map.

## Usage

 `1` ```area.map(m, regions = ".", sqmi=TRUE, ...) ```

## Arguments

 `m` a map object containing named polygons (created with `fill = TRUE`). `regions` a character vector naming one of more regions, as in `map`. `sqmi` If `TRUE`, measure area in square miles. Otherwise keep the units of `m`. `...` additional arguments to `match.map`

## Details

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.

## Value

a named vector of region areas.

## NOTE

The `sqmi` option assumes the coordinates have been projected with the mapproject function.

## Author(s)

Tom Minka

`area.polygon`, `apply.polygon`
 ``` 1 2 3 4 5 6 7 8 9 10 11 12``` ```# 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") } ```