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Measure how much ground a polygon covers, in square metres and square kilometres, with holes subtracted and every part of a MultiPolygon summed. Use it after POST /v1/geometry/intersect to size an overlap, or on a boundary you already hold. Points and lines contribute zero rather than an error, so a mixed collection is safe to send. Not turf's area, which applies the spherical-excess formula to geodetic latitudes on a 6371008.8 m sphere and comes out 0.45% high at the equator and 0.85% low at 80 deg latitude. This maps latitude to authalic latitude on the WGS 84 authalic sphere first, landing within 5e-3% of GeographicLib's geodesic area on parcel-scale rings and converging to 1e-6% as ring edges are densified. Ring edges are treated as straight in longitude/latitude, per RFC 7946 and turf; a geodesic-edge area differs on long slanted edges, by 1.9% on a 1.7-degree triangle. Winding order is ignored: ring 0 is the exterior and the rest are holes. Input is EPSG:4326 only. Limits: 100000 positions across the whole document, 1 MiB request body.