Set the map projection (CRS)

This function sets the map projection. It can also be set via tm_shape(), but tm_crs is generally recommended for most cases. It can also be determined automatically (see details); however, this is still work-in-progress.

Usage

tm_crs(crs = NA, property = NA)

Arguments

crs

Map projection (CRS). Can be set to an crs object (see sf::st_crs()), a proj4string, an EPSG number, the value "auto" (automatic crs recommendation), or one the the following generic projections: c("laea", "aeqd", "utm", "pconic", "eqdc", "stere"). See details.

property

Which property should the projection have? One of: "global", "area" (equal-area), "distance" (equidistant), "shape" (conformal). Only applicable if crs = "auto". See details.

Details

The map projection (crs) determines in which coordinate system the spatial object is processed and plotted. See vignette about CRS. The crs can be specified in two places: 1) tm_shape() and tm_crs(). In both cases, the map is plotted into the specified crs. The difference is that in the first option, the crs is also taken into account in spatial transformation functions, such as the calculation of centroids and cartograms. In the second option, the crs is only used in the plotting phase.

The automatic crs recommendation (which is still work-in-progress) is the following:

Property	Recommendation
global (for world maps)	A pseudocylindrical projection tmap option crs_global, by default "eqearth (Equal Eearth). See https://r-tmap.github.io/tmap/articles/41_advanced_crs.html for more options
area (equal area)	Lambert Azimuthal Equal Area (laea)
distance (equidistant)	Azimuthal Equidistant (aeqd)
shape (conformal)	Stereographic (stere)

For further info about the available "generic" projects see: for utm: https://proj.org/en/9.4/operations/projections/utm.html for laea: https://proj.org/en/9.4/operations/projections/laea.html for aeqd: https://proj.org/en/9.4/operations/projections/aeqd.html for pconic: https://proj.org/en/9.4/operations/projections/pconic.html for eqdc: https://proj.org/en/9.4/operations/projections/eqdc.html

Note

Plans are to migrate the functionality regarding generic crs and automatic crs recommendation to a separate package.

See also

vignette about CRS

Examples

SA = World[World$continent == "South America", ]

# latlon coordinates (WGS84)
tm_shape(SA) +
  tm_polygons() +
  tm_graticules() +
  tm_crs(4326)


tm_list = lapply(c("global", "area", "distance", "shape"), FUN = function(property) {
  tm_shape(SA) +
    tm_polygons() +
    tm_graticules() +
    tm_crs(property = property)  +
  tm_title(property)
})

tmap_arrange(tm_list, nrow = 1)