Calculate densities

Transpose quantitative variables to densitiy variables, which are often needed for choroplets. For example, the colors of a population density map should correspond population density counts rather than absolute population numbers.

Usage

calc_densities(
  shp,
  var,
  target = "metric",
  total.area = NULL,
  suffix = NA,
  drop = TRUE
)

Arguments

shp

a shape object, i.e., an sf object or a SpatialPolygons(DataFrame) from the sp package.

var

name(s) of a qualtity variable name contained in the shp data

target

the target unit, see approx_areas. Density values are calculated in var/target^2.

total.area

total area size of shp in number of target units (defined by unit), approx_areas.

suffix

character that is appended to the variable names. The resulting names are used as column names of the returned data.frame. By default, _sq_<target>, where target corresponds to the target unit, e.g. _sq_km

drop

boolean that determines whether an one-column data-frame should be returned as a vector

Value

Vector or data.frame (depending on whether length(var)==1 with density values.

Examples

if (require(tmap) && packageVersion("tmap") >= "3.99") {
    data(NLD_muni)

    NLD_muni_pop_per_km2 <- calc_densities(NLD_muni,
        target = "km km", var = c("population", "dwelling_total"))
    NLD_muni <- sf::st_sf(data.frame(NLD_muni, NLD_muni_pop_per_km2))

    tm_shape(NLD_muni) +
      tm_polygons(c("population_km.2", "dwelling_total_km.2"),
                  fill.legend = list(tm_legend(expression("Population per " * km^2)), tm_legend(expression("Dwellings per " * km^2)))) +
    tm_facets(free.scales = TRUE) +
        tm_layout(panel.show = FALSE)
}