PostGIS  2.3.7dev-r@@SVN_REVISION@@
double lwgeom_area_sphere ( const LWGEOM lwgeom,
const SPHEROID spheroid 
)

Calculate the area of an LWGEOM.

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 1938 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().

1939 {
1940  int type;
1941  double radius2 = spheroid->radius * spheroid->radius;
1942 
1943  assert(lwgeom);
1944 
1945  /* No area in nothing */
1946  if ( lwgeom_is_empty(lwgeom) )
1947  return 0.0;
1948 
1949  /* Read the geometry type number */
1950  type = lwgeom->type;
1951 
1952  /* Anything but polygons and collections returns zero */
1953  if ( ! ( type == POLYGONTYPE || type == MULTIPOLYGONTYPE || type == COLLECTIONTYPE ) )
1954  return 0.0;
1955 
1956  /* Actually calculate area */
1957  if ( type == POLYGONTYPE )
1958  {
1959  LWPOLY *poly = (LWPOLY*)lwgeom;
1960  int i;
1961  double area = 0.0;
1962 
1963  /* Just in case there's no rings */
1964  if ( poly->nrings < 1 )
1965  return 0.0;
1966 
1967  /* First, the area of the outer ring */
1968  area += radius2 * ptarray_area_sphere(poly->rings[0]);
1969 
1970  /* Subtract areas of inner rings */
1971  for ( i = 1; i < poly->nrings; i++ )
1972  {
1973  area -= radius2 * ptarray_area_sphere(poly->rings[i]);
1974  }
1975  return area;
1976  }
1977 
1978  /* Recurse into sub-geometries to get area */
1979  if ( type == MULTIPOLYGONTYPE || type == COLLECTIONTYPE )
1980  {
1981  LWCOLLECTION *col = (LWCOLLECTION*)lwgeom;
1982  int i;
1983  double area = 0.0;
1984 
1985  for ( i = 0; i < col->ngeoms; i++ )
1986  {
1987  area += lwgeom_area_sphere(col->geoms[i], spheroid);
1988  }
1989  return area;
1990  }
1991 
1992  /* Shouldn't get here. */
1993  return 0.0;
1994 }
#define POLYGONTYPE
Definition: liblwgeom.h:86
Datum area(PG_FUNCTION_ARGS)
double radius
Definition: liblwgeom.h:317
double lwgeom_area_sphere(const LWGEOM *lwgeom, const SPHEROID *spheroid)
Calculate the area of an LWGEOM.
Definition: lwgeodetic.c:1938
LWGEOM ** geoms
Definition: liblwgeom.h:508
POINTARRAY ** rings
Definition: liblwgeom.h:456
int nrings
Definition: liblwgeom.h:454
#define MULTIPOLYGONTYPE
Definition: liblwgeom.h:89
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:1714
uint8_t type
Definition: liblwgeom.h:395
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:1310
#define COLLECTIONTYPE
Definition: liblwgeom.h:90

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