coord_quickmap: Cartesian coordinates with an aspect ratio approximating Mercator projection.

Description

The represenation of a portion of the earth, wich 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 correctedness in the projection.

Usage

coord_quickmap(xlim = NULL, ylim = NULL)

Arguments

xlim

limits for the x axis

ylim

limits for the y axis

Examples

Run this code

# ensures that the ranges of axes are equal to the specified ratio by
# adjusting the plot aspect ratio

if (require("maps")) {
# Create a lat-long dataframe from the maps package
nz <- map_data("nz")
# Prepare a plot of the map
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()
}

# Resize the plot to see that the specified aspect ratio is maintained

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