Geodesic area and perimeter

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

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 an exterior ring must run counter-clockwise. A clockwise ring names the rest of the earth instead, and ga_unsigned_area_geodesic() returns a value near 5.1e14. Shapefiles wind clockwise, so orient with ga_orient() first or read the sign off ga_signed_area_geodesic(), which is unaffected.

References

GeodesicArea

See also

Other area: 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

# nc winds clockwise, so orient 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