Computes the EOO as the area of the convex hull (minimum convex polygon)
enclosing all occurrence points, following the IUCN Red List guidelines and
the approach used by the ConR package. The hull is built in
longitude/latitude, its edges are densified with st_segmentize() so
that they follow great circles, and the area is measured directly on the
WGS84 ellipsoid (a "spheroid" measurement).
Arguments
- points
An
sfof POINT geometries (one species). Useread_occurrencesto produce it.- segmentize_km
Numeric edge length, in kilometres, used to densify the hull edges along great circles (default
20, matching ConR).
Value
A list with area_km2 (numeric), n_records (integer),
n_unique (distinct coordinates), hull (the hull as an
sfc in WGS84, or NULL when the EOO is undefined) and
crs_laea (a data-centred equal-area proj string, kept for mapping).
Details
Degenerate cases are handled so the function never errors:
With fewer than three unique points a polygon cannot be built and the EOO is
NA.When the unique points are perfectly collinear (they would form a line of area zero) a small amount of noise is added to the coordinates (
jitter), exactly as in ConR, so that a valid polygon can still be produced.If, for any other reason, a valid polygon still cannot be built, the EOO is returned as
NArather than throwing an error.
Examples
f <- system.file("extdata", "example_occurrences.csv", package = "mappingAS")
occ <- read_occurrences(f)
sp1 <- occ[occ$species == occ$species[1], ]
calc_eoo(sp1)$area_km2
#> [1] 2468.019