areaPolygon {geosphere}

Area of a longitude/latitude polygon

Description

Compute the area of a polygon in angular coordinates (longitude/latitude) on an ellipsoid.

Usage

## S4 method for signature 'matrix'
areaPolygon(x, a=6378137, f=1/298.257223563, ...)

## S4 method for signature 'SpatialPolygons'
areaPolygon(x, a=6378137, f=1/298.257223563, ...)

Arguments

x

longitude/latitude of the points forming a polygon; Must be a matrix or data.frame of 2 columns (first one is longitude, second is latitude) or a SpatialPolygons* object

a

major (equatorial) radius of the ellipsoid

f

ellipsoid flattening. The default value is for WGS84

...

Additional arguments. None implemented

Value

area in square meters

Note

Use raster::area for polygons that have a planar (projected) coordinate reference system.

Author(s)

This function calls GeographicLib code by C.F.F. Karney

References

C.F.F. Karney, 2013. Algorithms for geodesics, J. Geodesy 87: 43-55. doi:10.1007/s00190-012-0578-z. Addenda: https://geographiclib.sourceforge.io/geod-addenda.html. Also see https://geographiclib.sourceforge.io/

See Also

centroid, perimeter

Examples

p <- rbind(c(-180,-20), c(-140,55), c(10, 0), c(-140,-60), c(-180,-20))
areaPolygon(p)

# Be careful with very large polygons, as they may not be what they seem!
# For example, if you wanted a polygon to compute the area equal to about 1/4 of the ellipsoid
# this won't work:
b <- matrix(c(-180, 0, 90, 90, 0, 0, -180, 0), ncol=2, byrow=TRUE)
areaPolygon(b)
# Becausee the shortest path between (-180,0) and (0,0) is 
# over one of the poles, not along the equator!
# Inserting a point along the equator fixes that
b <- matrix(c(-180, 0, 0, 0, -90,0, -180, 0), ncol=2, byrow=TRUE)
areaPolygon(b)