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), absolute numbers based on the polygon coordinates, proportional numbers, or normalized numbers.

Usage

approx_areas(shp, target = "metric", orig = NA, to = NA, total.area = NA, show.warnings = TRUE)

Arguments

shp

shape object, i.e., a SpatialPolygons(DataFrame), or an sf object that can be coerced as such.

target

target unit, one of

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

orig

original unit, i.e. by which the coordinates are defined. By default, the value is taken from the crs projection string defined in shp. Not needed for non-projected shapes, since their areas are determined in another way (see details).

multiplier used as follows: orig * to = target. Only needed when orig or target is unknown. For instance, if target is set to "hm" (hectameter), and orig is "m", then to should be 100, meaning 1 hectameter equals 100 meters.

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.

show.warnings

should warnings be shown?

Value

Numeric vector of area sizes. An attribute called unit is added to specify the used target units, which is especially useful when units were set to metric or imperial.

Details

Note that this method 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.

For projected shapes, gArea is used, and for unprojected shapes, areaPolygon.

Examples

Run this code

## Not run: 
# if (require(tmap)) {
#     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))
# 
# 
#     # 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),
#              units=paste0(attr(x, "unit")))
#     }
# 
#     # area of the polygons
#     approx_areas(NLD_muni) %>% summary_areas()
# 
#     # area of the polygons, adjusted corrected for a specified total area size
#     approx_areas(NLD_muni, total.area=33893) %>% summary_areas()
# 
#     # proportional area of the polygons
#     approx_areas(NLD_muni, target = "prop") %>% summary_areas()
# 
#     # area in squared miles
#     approx_areas(NLD_muni, target = "mi") %>% summary_areas()
# 
#     # area of the polygons when unprojected
#     approx_areas(NLD_muni %>% set_projection(projection="longlat")) %>% summary_areas()
# }
# ## End(Not run)