Skip to contents

Tissot's indicatrix: what a projection does to the ground

Source: R/projections.R
tissot_map.Rd

Draws Tissot indicatrices – small circles of equal ground radius, placed on a graticule and projected with everything else. On an equal-area projection every ellipse encloses the same area (though shapes shear); on a conformal projection every ellipse stays circular but sizes explode. It is the standard cartographic device for showing what a projection costs, and it makes the package's "honest maps" claim visible instead of asserted.

Usage

tissot_map(
  projection = "equal_earth",
  spacing = 30,
  radius_km = 500,
  max_lat = 75,
  fill = "#B2182B",
  color = "grey20"
)

Arguments

projection

Projection to illustrate (see projection_info()).

spacing

Degrees between indicatrices (default 30).

radius_km

Ground radius of each circle in kilometres (default 500).

max_lat

Absolute latitude limit for the circle centres (default 75; the poles are singular in most projections).

fill, color

Fill and outline colour for the ellipses.

Value

A ggplot object: the world outline with indicatrices on top.

See also

projection_info(), projection_compare()

Examples

# \donttest{
if (requireNamespace("sf", quietly = TRUE) &&
    requireNamespace("rnaturalearth", quietly = TRUE)) {
  tissot_map("mercator")      # circles stay round, and grow enormously
  tissot_map("equal_earth")   # equal areas, sheared shapes
}

# }