coord_map projects a portion of the earth, which is approximately spherical, onto a flat 2D plane using any projection defined by the mapproj package. Map projections do not, in general, preserve straight lines, so this requires considerable computation. coord_quickmap is a quick approximation that does preserve straight lines. It works best for smaller areas closer to the equator.

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

coord_quickmap(xlim = NULL, ylim = NULL, expand = TRUE,
clip = "on")

Arguments

projection projection to use, see mapproj::mapproject() for list Other arguments passed on to mapproj::mapproject(). Use ... for named parameters to the projection, and parameters for unnamed parameters. ... is ignored if the parameters argument is present. 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 mapproj::mapproject() for more information. Manually specific x/y limits (in degrees of longitude/latitude) Should drawing be clipped to the extent of the plot panel? A setting of "on" (the default) means yes, and a setting of "off" means no. For details, please see coord_cartesian(). 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.

Details

In general, map projections must 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 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, and is much faster (particularly for complex plots like geom_tile()) at the expense of correctness.

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 } if (require("maps")) { # With correct mercator projection nzmap + coord_map() } if (require("maps")) { # With the aspect ratio approximation nzmap + coord_quickmap() } if (require("maps")) { # Other projections nzmap + coord_map("azequalarea", orientation = c(-36.92, 174.6, 0)) } if (require("maps")) { states <- map_data("state") usamap <- ggplot(states, aes(long, lat, group = group)) + geom_polygon(fill = "white", colour = "black") # Use cartesian coordinates usamap } if (require("maps")) { # With mercator projection usamap + coord_map() } if (require("maps")) { # See ?mapproject for coordinate systems and their parameters usamap + coord_map("gilbert") } if (require("maps")) { # 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") } if (require("maps")) { usamap + coord_map("conic", lat0 = 30) } if (require("maps")) { usamap + coord_map("bonne", lat0 = 50) } # NOT RUN { if (require("maps")) { # 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") } if (require("maps")) { # Looking up up at South Pole worldmap + coord_map("ortho", orientation = c(-90, 0, 0)) } if (require("maps")) { # Centered on New York (currently has issues with closing polygons) worldmap + coord_map("ortho", orientation = c(41, -74, 0)) } # }