PostGIS  2.2.7dev-r@@SVN_REVISION@@
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 1927 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, and LWGEOM::type.

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

1928 {
1929  int type;
1930  double radius2 = spheroid->radius * spheroid->radius;
1931 
1932  assert(lwgeom);
1933 
1934  /* No area in nothing */
1935  if ( lwgeom_is_empty(lwgeom) )
1936  return 0.0;
1937 
1938  /* Read the geometry type number */
1939  type = lwgeom->type;
1940 
1941  /* Anything but polygons and collections returns zero */
1942  if ( ! ( type == POLYGONTYPE || type == MULTIPOLYGONTYPE || type == COLLECTIONTYPE ) )
1943  return 0.0;
1944 
1945  /* Actually calculate area */
1946  if ( type == POLYGONTYPE )
1947  {
1948  LWPOLY *poly = (LWPOLY*)lwgeom;
1949  int i;
1950  double area = 0.0;
1951 
1952  /* Just in case there's no rings */
1953  if ( poly->nrings < 1 )
1954  return 0.0;
1955 
1956  /* First, the area of the outer ring */
1957  area += radius2 * ptarray_area_sphere(poly->rings[0]);
1958 
1959  /* Subtract areas of inner rings */
1960  for ( i = 1; i < poly->nrings; i++ )
1961  {
1962  area -= radius2 * ptarray_area_sphere(poly->rings[i]);
1963  }
1964  return area;
1965  }
1966 
1967  /* Recurse into sub-geometries to get area */
1968  if ( type == MULTIPOLYGONTYPE || type == COLLECTIONTYPE )
1969  {
1970  LWCOLLECTION *col = (LWCOLLECTION*)lwgeom;
1971  int i;
1972  double area = 0.0;
1973 
1974  for ( i = 0; i < col->ngeoms; i++ )
1975  {
1976  area += lwgeom_area_sphere(col->geoms[i], spheroid);
1977  }
1978  return area;
1979  }
1980 
1981  /* Shouldn't get here. */
1982  return 0.0;
1983 }
#define POLYGONTYPE
Definition: liblwgeom.h:72
Datum area(PG_FUNCTION_ARGS)
double radius
Definition: liblwgeom.h:302
double lwgeom_area_sphere(const LWGEOM *lwgeom, const SPHEROID *spheroid)
Calculate the area of an LWGEOM.
Definition: lwgeodetic.c:1927
LWGEOM ** geoms
Definition: liblwgeom.h:493
POINTARRAY ** rings
Definition: liblwgeom.h:441
int nrings
Definition: liblwgeom.h:439
#define MULTIPOLYGONTYPE
Definition: liblwgeom.h:75
double ptarray_area_sphere(const POINTARRAY *pa)
Returns the area of the ring (ring must be closed) in square radians (surface of the sphere is 4*PI)...
Definition: lwgeodetic.c:1703
uint8_t type
Definition: liblwgeom.h:380
int lwgeom_is_empty(const LWGEOM *geom)
Return true or false depending on whether a geometry is an "empty" geometry (no vertices members) ...
Definition: lwgeom.c:1297
#define COLLECTIONTYPE
Definition: liblwgeom.h:76

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