coord_map {ggplot2} | R Documentation

## Map projections.

### Description

The representation of a portion of the earth, which is approximately spherical, onto a flat 2D plane requires a projection. This is what `coord_map` does. These projections account for the fact that the actual length (in km) of one degree of longitude varies between the equator and the pole. Near the equator, the ratio between the lengths of one degree of latitude and one degree of longitude is approximately 1. Near the pole, it is tends towards infinity because the length of one degree of longitude tends towards 0. For regions that span only a few degrees and are not too close to the poles, setting the aspect ratio of the plot to the appropriate lat/lon ratio approximates the usual mercator projection. This is what `coord_quickmap` does. With `coord_map` all elements of the graphic have to be projected which is not the case here. So `coord_quickmap` has the advantage of being much faster, in particular for complex plots such as those using with `geom_tile`, at the expense of correctness in the projection. This coordinate system provides the full range of map projections available in the mapproj package.

### Usage

    coord_map(projection = "mercator", ..., orientation = NULL, xlim = NULL,
      ylim = NULL)

    coord_quickmap(xlim = NULL, ylim = NULL, expand = TRUE)

### Arguments

`projection` | projection to use, see `mapproject` for list
`...` | other arguments passed on to `mapproject`
`orientation` | projection orientation, which defaults to `c(90, 0, mean(range(x)))`. This is not optimal for many projections, so you will have to supply your own. See `mapproject` for more information.
`xlim` | manually specific x limits (in degrees of longitude)
`ylim` | manually specific y limits (in degrees of latitude)
`expand` | If `TRUE`, the default, adds a small expansion factor to the limits to ensure that data and axes don't overlap. If `FALSE`, limits are taken exactly from the data or `xlim`/`ylim`.

### Examples

    if (require("maps")) {
    nz <- map_data("nz")
    # Prepare a map of NZ
    nzmap <- ggplot(nz, aes(x = long, y = lat, group = group)) +
      geom_polygon(fill = "white", colour = "black")

    # Plot it in cartesian coordinates
    nzmap
    # With correct mercator projection
    nzmap + coord_map()
    # With the aspect ratio approximation
    nzmap + coord_quickmap()

    # Other projections
    nzmap + coord_map("cylindrical")
    nzmap + coord_map("azequalarea",orientation=c(-36.92,174.6,0))

    states <- map_data("state")
    usamap <- ggplot(states, aes(long, lat, group = group)) +
      geom_polygon(fill = "white", colour = "black")

    # Use cartesian coordinates
    usamap
    # With mercator projection
    usamap + coord_map()
    usamap + coord_quickmap()
    # See ?mapproject for coordinate systems and their parameters
    usamap + coord_map("gilbert")
    usamap + coord_map("lagrange")

    # For most projections, you'll need to set the orientation yourself
    # as the automatic selection done by mapproject is not available to
    # ggplot
    usamap + coord_map("orthographic")
    usamap + coord_map("stereographic")
    usamap + coord_map("conic", lat0 = 30)
    usamap + coord_map("bonne", lat0 = 50)

    # World map, using geom_path instead of geom_polygon
    world <- map_data("world")
    worldmap <- ggplot(world, aes(x=long, y=lat, group=group)) +
      geom_path() +
      scale_y_continuous(breaks = (-2:2) * 30) +
      scale_x_continuous(breaks = (-4:4) * 45)

    # Orthographic projection with default orientation (looking down at North pole)
    worldmap + coord_map("ortho")
    # Looking up up at South Pole
    worldmap + coord_map("ortho", orientation = c(-90, 0, 0))
    # Centered on New York (currently has issues with closing polygons)
    worldmap + coord_map("ortho", orientation = c(41, -74, 0))
    }

[Package ggplot2 version 2.1.0 Index]