Add Tissot's indicatrix to a map

This function adds Tissot's indicatrix to a map. Tissot's indicatrix is a mathematical contrivance used in cartography to characterize local distortions due to map projection.

geom_tissot(
  mapping = ggplot2::aes(),
  data = NULL,
  na.rm = FALSE,
  show.legend = NA,
  inherit.aes = TRUE,
  centres = c(5, 5),
  radius = NULL,
  fill = "red",
  alpha = 0.7,
  ...
)

Arguments

mapping

Set of aesthetic mappings created by ggplot2::aes(). If specified and inherit.aes = TRUE (the default), it is combined with the default mapping at the top level of the plot. You must supply mapping if there is no plot mapping.

data

The data to be displayed in this layer. There are two options: A sf object or a SpatRaster object

na.rm

If FALSE, the default, missing values are removed with a warning. If TRUE, missing values are silently removed.

show.legend

logical. Should this layer be included in the legends? NA, the default, includes if any aesthetics are mapped. FALSE never includes, and TRUE always includes. It can also be a named logical vector to finely select the aesthetics to display.

inherit.aes

If FALSE, overrides the default aesthetics, rather than combining with them. This is most useful for helper functions that define both data and aesthetics and shouldn't inherit behaviour from the default plot specification, e.g. ggplot2::geom_point().

centres

Either a list with elements "lng" and "lat" giving the longitudes and latitudes of the grid of centres for the Tissot's indicatrix, or a vector of length 2 with the number of rows and columns to generate such a grid automatically. The latter is passed to pretty() to get pretty breaks, and the exact number of Tissot's circles might differ from the input numbers (see pretty() for details. It defaults to c(5,5).

radius

The radius of the circles (see sf::st_buffer() for an explanation of how units are created; we recommend that you explicitly state your using with the units::as_units(), e.g units::as_units(100, "km")

fill

The fill colour of the circles

alpha

The transparency of the circles

...

Other arguments passed on to ggplot2::geom_sf()

Value

A ggplot2 object

Examples

# load required packages
library(rnaturalearth)
library(sf)
#> Linking to GEOS 3.12.1, GDAL 3.8.4, PROJ 9.4.0; sf_use_s2() is TRUE
library(ggplot2)
s_america_sf <- ne_countries(continent = "South America", returnclass = "sf")
s_am_equal_area <- suggest_crs(s_america_sf, distortion = "equal_area")
#> To reduce overall distortion on the map, one can also compress the map in the north-south direction (with a factor s) and expand the map in the east-west direction (with a factor 1 / s). The factor s can be determined with a trial-and-error approach comparing the distortion patterns along the centre and at the border of the map.
ggplot(data = s_america_sf) +
 geom_sf() +
 geom_tissot() +
 coord_sf(crs = s_am_equal_area$proj4) +
 theme_minimal()