1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
|
\author{Tom Minka}
\name{area.map}
\alias{area.map}
\title{
Area of projected map regions
}
\usage{
area.map(m, regions = ".", sqmi=TRUE, ...)
}
\description{Computes the areas of regions in a projected map.}
\arguments{
\item{m}{a map object containing named polygons (created with
\code{fill = TRUE}).}
\item{regions}{a character vector naming one of more regions, as in
\code{\link{map}}.}
\item{sqmi}{If \code{TRUE}, measure area in square miles. Otherwise
keep the units of \code{m}.}
\item{...}{additional arguments to \code{\link{match.map}}}
}
\details{
The area of each matching region in the map is computed, and
regions which match the same element of \code{regions} have their
areas combined. Each region is assumed planar, with vertices
specified by the \code{x} and \code{y} components of the map object.
The correct use of this function is to first use \code{\link{map}} to
create polygons and project the coordinates onto a plane, then apply
\code{area.map} to compute the area of the projected regions.
If the projection is area-preserving (such as \code{albers}),
then these areas will match the area on the globe, up to a
constant. To get an absolute area in square miles, the \code{sqmi}
option will scale the result, depending on the projection.
The coordinates from \code{\link{map}} are affected by
its \code{resolution} argument, so use \code{resolution=0} for the most
accurate areas.
}
\section{NOTE}{
The \code{sqmi} option assumes the coordinates have been projected
with the \link[mapproj]{mapproject} function.
}
\value{
a named vector of region areas.
}
\seealso{\code{area.polygon}, \code{apply.polygon}}
\examples{
# 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")
}
}
\keyword{dplot}
|
