Compute the concave hull of geometries

Returns a polygon enclosing each geometry, and tightens that polygon toward the geometry's own shape. With smaller concavity values, ga_concave_hull() produces a tighter, more concave hull; with larger values, it approaches the convex hull.

Usage

ga_concave_hull(geometry, concavity, length_threshold)

Arguments

geometry

a GeoArrow multipoint, linestring, multilinestring, polygon, or multipolygon array

concavity

the concavity coefficient; length 1 or the same length as geometry. geo uses 2 by default

length_threshold

the length below which ga_concave_hull() stops refining an edge; length 1 or the same length as geometry. geo uses 0 by default

Value

a GeoArrow polygon array

Details

Accepts multipoints, linestrings, multilinestrings, polygons, and multipolygons. geo does not define a concave hull for a single point.

References

ConcaveHull

See also

Other boundary: ga_convex_hull(), ga_envelope(), ga_extremes(), ga_minimum_rotated_rect()

Examples

fp <- system.file("shape/nc.shp", package = "sf")
nc <- as.data.frame(read_shapefile(fp))

# with lower concavity, ga_concave_hull() tightens the hull onto the shape
loose <- ga_concave_hull(nc$geometry, concavity = 2, length_threshold = 0)
tight <- ga_concave_hull(nc$geometry, concavity = 0.5, length_threshold = 0)

head(as.vector(ga_unsigned_area(loose)), 3)
#> [1] 0.11502123 0.06249588 0.12857049
head(as.vector(ga_unsigned_area(tight)), 3)
#> [1] 0.11428350 0.02801177 0.08427341