Map projections: coord_map

Mix.install([
  {:giggly, path: Path.join(__DIR__, "..")},
  {:kino, "~> 0.19"}
  ])

About this notebook

This notebook follows the examples on ggplot2's reference page for coord_map(), which covers coord_quickmap() too, in the order the page gives them. Each example shows ggplot2's code first and then Giggly's. An example Giggly cannot draw yet keeps its ggplot2 code and names what it is waiting for, so the page is also a list of what remains.

coord_map() projects part of the earth, which is roughly a sphere, onto a flat plane, with any projection the mapproj package defines. A projection need not keep straight lines straight, so this takes a good deal of computation. coord_quickmap() approximates it more quickly by fixing the aspect ratio to the ratio between the lengths of a degree of latitude and a degree of longitude, which keeps lines straight and works best for small areas near the equator. Both are superseded by coord_sf(). Giggly has none of these yet, nor the map data the examples draw.

New Zealand in Cartesian coordinates

R's if (require("maps")) runs an example only when the maps package, which holds the map data, is installed.

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
}

Not in Giggly yet: map data from the maps package and geom_polygon().

The Mercator projection

if (require("maps")) {
# With correct mercator projection
nzmap + coord_map()
}

Not in Giggly yet: map data from the maps package, geom_polygon(), and coord_map().

The aspect ratio approximation

if (require("maps")) {
# With the aspect ratio approximation
nzmap + coord_quickmap()
}

Not in Giggly yet: map data from the maps package, geom_polygon(), and coord_quickmap().

Another projection

if (require("maps")) {
# Other projections
nzmap + coord_map("azequalarea", orientation = c(-36.92, 174.6, 0))
}

Not in Giggly yet: map data from the maps package, geom_polygon(), and coord_map() with its projections and its orientation option.

The United States in Cartesian coordinates

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
}

Not in Giggly yet: map data from the maps package and geom_polygon().

The United States in the Mercator projection

if (require("maps")) {
# With mercator projection
usamap + coord_map()
}

Not in Giggly yet: map data from the maps package, geom_polygon(), and coord_map().

Other projections

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 in Giggly yet: map data from the maps package, geom_polygon(), and coord_map() with its projections and their parameters.

A world map seen from above the North Pole

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")
}

Not in Giggly yet: map data from the maps package and coord_map() with its projections.

Other orientations

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))
}

Not in Giggly yet: map data from the maps package, and coord_map() with its projections and its orientation option.