PostGIS  2.2.7dev-r@@SVN_REVISION@@
double ptarray_signed_area ( const POINTARRAY pa)

Returns the area in cartesian units.

Area is negative if ring is oriented CCW, positive if it is oriented CW and zero if the ring is degenerate or flat. http://en.wikipedia.org/wiki/Shoelace_formula

Definition at line 995 of file ptarray.c.

References getPoint2d_cp(), POINTARRAY::npoints, pixval::x, POINT2D::x, and POINT2D::y.

Referenced by lwpoly_area(), ptarray_isccw(), and test_ptarray_signed_area().

996 {
997  const POINT2D *P1;
998  const POINT2D *P2;
999  const POINT2D *P3;
1000  double sum = 0.0;
1001  double x0, x, y1, y2;
1002  int i;
1003 
1004  if (! pa || pa->npoints < 3 )
1005  return 0.0;
1006 
1007  P1 = getPoint2d_cp(pa, 0);
1008  P2 = getPoint2d_cp(pa, 1);
1009  x0 = P1->x;
1010  for ( i = 2; i < pa->npoints; i++ )
1011  {
1012  P3 = getPoint2d_cp(pa, i);
1013  x = P2->x - x0;
1014  y1 = P3->y;
1015  y2 = P1->y;
1016  sum += x * (y2-y1);
1017 
1018  /* Move forwards! */
1019  P1 = P2;
1020  P2 = P3;
1021  }
1022  return sum / 2.0;
1023 }
int npoints
Definition: liblwgeom.h:355
double x
Definition: liblwgeom.h:312
const POINT2D * getPoint2d_cp(const POINTARRAY *pa, int n)
Returns a POINT2D pointer into the POINTARRAY serialized_ptlist, suitable for reading from...
Definition: lwgeom_api.c:472
double y
Definition: liblwgeom.h:312
tuple x
Definition: pixval.py:53

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