tmaptools (version 3.3)

approx_areas: Approximate area sizes of the shapes

Description

Approximate the area sizes of the polygons in real-world area units (such as sq km or sq mi), proportional numbers, or normalized numbers. Also, the areas can be calibrated to a prespecified area total. This function is a convenient wrapper around st_area.

Usage

approx_areas(shp, target = "metric", total.area = NULL)

Value

Numeric vector of area sizes (class units).

Arguments

shp

shape object, i.e., an sf or sp object.

target

target unit, one of

"prop":

Proportional numbers. In other words, the sum of the area sizes equals one.

"norm":

Normalized numbers. All area sizes are normalized to the largest area, of which the area size equals one.

"metric" (default):

Output area sizes will be either "km" (kilometer) or "m" (meter) depending on the map scale

"imperial":

Output area sizes will be either "mi" (miles) or "ft" (feet) depending on the map scale

other:

Predefined values are "km^2", "m^2", "mi^2", and "ft^2". Other values can be specified as well, in which case to is required).

These units are the output units. See orig for the coordinate units used by the shape shp.

total.area

total area size of shp in number of target units (defined by target). Useful if the total area of the shp differs from a reference total area value. For "metric" and "imperial" units, please provide the total area in squared kilometers respectively miles.

Details

Note that the method of determining areas is an approximation, since it depends on the used projection and the level of detail of the shape object. Projections with equal-area property are highly recommended. See https://en.wikipedia.org/wiki/List_of_map_projections for equal area world map projections.

See Also

approx_distances

Examples

if (require(tmap)) {
    data(NLD_muni)

    NLD_muni$area <- approx_areas(NLD_muni, total.area = 33893)

    tm_shape(NLD_muni) +
        tm_bubbles(size="area",
                   size.legend = tm_legend(title = expression("Area in " * km^2)))


    # function that returns min, max, mean and sum of area values
    summary_areas <- function(x) {
        list(min_area=min(x),
             max_area=max(x),
             mean_area=mean(x),
             sum_area=sum(x))
    }

    # area of the polygons
    summary_areas(approx_areas(NLD_muni))

    # area of the polygons, adjusted corrected for a specified total area size
    summary_areas(approx_areas(NLD_muni, total.area=33893))

    # proportional area of the polygons
    summary_areas(approx_areas(NLD_muni, target = "prop"))

    # area in squared miles
    summary_areas(approx_areas(NLD_muni, target = "mi mi"))

    # area of the polygons when unprojected
    summary_areas(approx_areas(sf::st_transform(NLD_muni, crs = 4326)))
}