Description Usage Arguments Value Note Author(s) References See Also Examples

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

1 2 3 4 5 | ```
## 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, ...)
``` |

`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 |

area in square meters

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

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

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

1 2 3 4 5 6 7 8 9 10 11 12 13 | ```
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)
``` |

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