R: Draw a latitude/longitude grid on a projected map

map.grid {mapproj}

R Documentation

Draw a latitude/longitude grid on a projected map

Description

Draws a grid on an existing map.

Usage

map.grid(lim,nx=9,ny=9,labels=TRUE,pretty=TRUE,cex,col,lty,font,...)

Arguments

`lim`	a vector of 4 numbers specifying limits: `c(lon.low, lon.high, lat.low, lat.high)`. `lim` can also be a list with a component named `range`, such as the result of `map`, from which limits are taken.
`nx,ny`	the desired number of equally-spaced longitude and latitude lines
`labels`	logical to indicate if grid lines should be labeled with longitude/latitude values.
`pretty`	If `TRUE`, grid lines will be placed at round numbers.
`cex`
`col`
`lty`
`font`
`...`	additional arguments passed to `lines` and `text`, e.g. `col` to change the color of the grid and `lty` to change the line type.

Value

Equally-spaced lines of constant longitude and lines of constant latitude are superimposed on the current map, using the current projection. These lines will appear curved under most projections, and give an idea of how the projection works.

Examples

library(maps)
m <- map("usa",plot=FALSE)
map("usa",project="albers",par=c(39,45))
map.grid(m)

# get unprojected world limits
m <- map('world',plot=FALSE)

# center on NYC
map('world',proj='azequalarea',orient=c(41,-74,0))
map.grid(m,col=2)
points(mapproject(list(y=41,x=-74)),col=3,pch="x",cex=2)

map('world',proj='orth',orient=c(41,-74,0))
map.grid(m,col=2,nx=6,ny=5,label=FALSE,lty=2)
points(mapproject(list(y=41,x=-74)),col=3,pch="x",cex=2)

# center on Auckland
map('world',proj='orth',orient=c(-36.92,174.6,0))
map.grid(m,col=2,label=FALSE,lty=2)
points(mapproject(list(y=-36.92,x=174.6)),col=3,pch="x",cex=2)

m <- map('nz')
# center on Auckland
map('nz',proj='azequalarea',orient=c(-36.92,174.6,0))
points(mapproject(list(y=-36.92,x=174.6)),col=3,pch="x",cex=2)
map.grid(m,col=2)