Geodesic area and perimeter

These functions use the geodesic formula for computing area and perimeter on an ellipsoidal model of the earth. They return area in square meters and perimeter in meters.

Usage

ga_signed_area_geodesic(x)

ga_unsigned_area_geodesic(x)

ga_perimeter_geodesic(x)

Arguments

x

a GeoArrow geometry array

Value

a double vector of area values in square meters, or perimeter values in meters

Details

geo assumes Simple Features winding, so it expects an exterior ring to run counter-clockwise. ga_unsigned_area_geodesic() reads a clockwise ring as the rest of the earth instead and returns a value near 5.1e14. For shapefiles, which wind clockwise, orient with ga_orient() first or read the sign off ga_signed_area_geodesic(), which does not have this problem.

References

GeodesicArea

See also

Other area: ga_perimeter_euclidean(), ga_signed_area(), ga_signed_area_cd()

Examples

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

head(as.vector(ga_signed_area_geodesic(nc$geometry)))
#> [1] -1137389166  -611077451 -1423490699  -694546681 -1520741303  -967728604
head(as.vector(ga_perimeter_geodesic(nc$geometry)))
#> [1] 141665.2 119928.6 160497.5 301517.1 211952.3 160891.0

# orient nc, which winds clockwise, before taking the unsigned area
ccw <- ga_orient(nc$geometry, "default")
head(as.vector(ga_unsigned_area_geodesic(ccw)))
#> [1] 1137389166  611077451 1423490699  694546681 1520741303  967728604