Map projections.

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))
}