lwgeom_area_sphere()

double lwgeom_area_sphere	(	const LWGEOM *	lwgeom,
		const SPHEROID *	spheroid
	)

Calculate the geodetic area of a lwgeom on the sphere.

The result will be multiplied by the average radius of the supplied spheroid.

Calculate the geodetic area of a lwgeom on the sphere.

Anything except POLYGON, MULTIPOLYGON and GEOMETRYCOLLECTION return zero immediately. Multi's recurse, polygons calculate external ring area and subtract internal ring area. A GBOX is required to calculate an outside point.

Definition at line 2027 of file lwgeodetic.c.

References area(), COLLECTIONTYPE, LWCOLLECTION::geoms, lwgeom_is_empty(), MULTIPOLYGONTYPE, LWCOLLECTION::ngeoms, LWPOLY::nrings, POLYGONTYPE, ptarray_area_sphere(), SPHEROID::radius, LWPOLY::rings, ovdump::type, and LWGEOM::type.

Referenced by geography_area(), geography_centroid_from_mpoly(), test_lwgeom_area_sphere(), and test_spheroid_area().

{
        int type;
        double radius2 = spheroid->radius * spheroid->radius;

        assert(lwgeom);

        /* No area in nothing */
        if ( lwgeom_is_empty(lwgeom) )
                return 0.0;

        /* Read the geometry type number */
        type = lwgeom->type;

        /* Anything but polygons and collections returns zero */
        if ( ! ( type == POLYGONTYPE || type == MULTIPOLYGONTYPE || type == COLLECTIONTYPE ) )
                return 0.0;

        /* Actually calculate area */
        if ( type == POLYGONTYPE )
        {
                LWPOLY *poly = (LWPOLY*)lwgeom;
                int i;
                double area = 0.0;

                /* Just in case there's no rings */
                if ( poly->nrings < 1 )
                        return 0.0;

                /* First, the area of the outer ring */
                area += radius2 * ptarray_area_sphere(poly->rings[0]);

                /* Subtract areas of inner rings */
                for ( i = 1; i < poly->nrings; i++ )
                {
                        area -= radius2 * ptarray_area_sphere(poly->rings[i]);
                }
                return area;
        }

        /* Recurse into sub-geometries to get area */
        if ( type == MULTIPOLYGONTYPE || type == COLLECTIONTYPE )
        {
                LWCOLLECTION *col = (LWCOLLECTION*)lwgeom;
                int i;
                double area = 0.0;

                for ( i = 0; i < col->ngeoms; i++ )
                {
                        area += lwgeom_area_sphere(col->geoms[i], spheroid);
                }
                return area;
        }

        /* Shouldn't get here. */
        return 0.0;
}

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