tmap (version 1.4-1)

approx_areas: Approximate area sizes of the shapes

Description

Approximate the area sizes of the polygons either in 1) absolute numbers based on the polygon coordinates, 2) proportional numbers, 3) normalized numbers and 4) squared units (e.g. kilometers).

Usage

approx_areas(shp, unit = "km", unit.size = 1000, total.area = NA)

Arguments

shp

shape object, i.e., a SpatialPolygons(DataFrame)

unit

one of

"abs":

Absolute numbers based on polygon coordinates.

"prop":

Proportional numbers. In other words, the total 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.

other:

For instance, "km", "m", or "miles". For this method, total.area or unit.size is required. In this case, the area sizes are returned in squared units, e.g., in squared kilometers.

The default method is "km".

unit.size

size of the unit in terms of coordinate units. The coordinate system of many projections is approximately in meters while thematic maps typically range many kilometers, so by default unit="km" and unit.size=1000 (meaning 1 kilometer equals 1000 coordinate units).

total.area

total area size of shp in number of squared units (by default kilometers). Useful if the total area of the shp differs from a reference total area value.

Value

Numeric vector of area sizes.

Details

To approximate the sizes in squared units, either the total.area or the unit.size is required. Note that this method is an approximation, since it depends on the used projection and the level of detail of the SpatialPolygons object. Projections with equal-area property are highly recommended.

Examples

data(NLD_muni)

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

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