PostGIS  2.5.2dev-r@@SVN_REVISION@@

◆ 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 1944 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().

1945 {
1946  int type;
1947  double radius2 = spheroid->radius * spheroid->radius;
1948 
1949  assert(lwgeom);
1950 
1951  /* No area in nothing */
1952  if ( lwgeom_is_empty(lwgeom) )
1953  return 0.0;
1954 
1955  /* Read the geometry type number */
1956  type = lwgeom->type;
1957 
1958  /* Anything but polygons and collections returns zero */
1959  if ( ! ( type == POLYGONTYPE || type == MULTIPOLYGONTYPE || type == COLLECTIONTYPE ) )
1960  return 0.0;
1961 
1962  /* Actually calculate area */
1963  if ( type == POLYGONTYPE )
1964  {
1965  LWPOLY *poly = (LWPOLY*)lwgeom;
1966  uint32_t i;
1967  double area = 0.0;
1968 
1969  /* Just in case there's no rings */
1970  if ( poly->nrings < 1 )
1971  return 0.0;
1972 
1973  /* First, the area of the outer ring */
1974  area += radius2 * ptarray_area_sphere(poly->rings[0]);
1975 
1976  /* Subtract areas of inner rings */
1977  for ( i = 1; i < poly->nrings; i++ )
1978  {
1979  area -= radius2 * ptarray_area_sphere(poly->rings[i]);
1980  }
1981  return area;
1982  }
1983 
1984  /* Recurse into sub-geometries to get area */
1985  if ( type == MULTIPOLYGONTYPE || type == COLLECTIONTYPE )
1986  {
1987  LWCOLLECTION *col = (LWCOLLECTION*)lwgeom;
1988  uint32_t i;
1989  double area = 0.0;
1990 
1991  for ( i = 0; i < col->ngeoms; i++ )
1992  {
1993  area += lwgeom_area_sphere(col->geoms[i], spheroid);
1994  }
1995  return area;
1996  }
1997 
1998  /* Shouldn't get here. */
1999  return 0.0;
2000 }
#define POLYGONTYPE
Definition: liblwgeom.h:86
Datum area(PG_FUNCTION_ARGS)
double radius
Definition: liblwgeom.h:320
uint32_t ngeoms
Definition: liblwgeom.h:509
uint32_t nrings
Definition: liblwgeom.h:457
unsigned int uint32_t
Definition: uthash.h:78
double lwgeom_area_sphere(const LWGEOM *lwgeom, const SPHEROID *spheroid)
Calculate the area of an LWGEOM.
Definition: lwgeodetic.c:1944
LWGEOM ** geoms
Definition: liblwgeom.h:511
POINTARRAY ** rings
Definition: liblwgeom.h:459
#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:1720
uint8_t type
Definition: liblwgeom.h:398
type
Definition: ovdump.py:41
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:1393
#define COLLECTIONTYPE
Definition: liblwgeom.h:90
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