GRASS GIS 7 Programmer's Manual  7.5.svn(2018)-r72387
area_poly2.c
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1 /*!
2  * \file lib/gis/area_poly2.c
3  *
4  * \brief GIS Library - Planimetric polygon area calculation routines.
5  *
6  * (C) 2001-2009 by the GRASS Development Team
7  *
8  * This program is free software under the GNU General Public License
9  * (>=v2). Read the file COPYING that comes with GRASS for details.
10  *
11  * \author Original author CERL
12  */
13
14 #include <grass/gis.h>
15
16 /*!
17  * \brief Calculates planimetric polygon area.
18  *
19  * \param x array of x values
20  * \param y array of y values
21  * \param n number of x,y pairs
22
23  * \return polygon area in map units
24  */
25 double G_planimetric_polygon_area(const double *x, const double *y, int n)
26 {
27  double x1, y1, x2, y2;
28  double area;
29
30  x2 = x[n - 1];
31  y2 = y[n - 1];
32
33  area = 0;
34  while (--n >= 0) {
35  x1 = x2;
36  y1 = y2;
37
38  x2 = *x++;
39  y2 = *y++;
40
41  area += (y2 + y1) * (x2 - x1);
42  }
43
44  if ((area /= 2.0) < 0.0)
45  area = -area;
46
47  return area;
48 }
double G_planimetric_polygon_area(const double *x, const double *y, int n)
Calculates planimetric polygon area.
Definition: area_poly2.c:25
#define x