GRASS GIS 7 Programmer's Manual  7.5.svn(2017)-r71915
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pole_in_poly.c
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1 /*!
2  \file lib/gis/pole_in_poly.c
3 
4  \brief GIS Library - Pole in polygon
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 CERL
12  */
13 
14 #include <grass/gis.h>
15 
16 static void mystats(double, double, double, double, double *, double *);
17 
18 /*!
19  * \brief Check if pole is in polygon
20  *
21  * For latitude-longitude coordinates, this routine determines if the polygon
22  * defined by the <i>n</i> coordinate vertices <i>x,y</i> contains one of the
23  * poles.
24  *
25  * <b>Note:</b> Use this routine only if the projection is PROJECTION_LL.
26  *
27  * \param x array of x coordinates
28  * \param y array of y coordinates
29  * \param n number of coordinates
30  *
31  * \return -1 if it contains the south pole
32  * \return 1 if it contains the north pole
33  * \return 0 if it contains neither pole.
34  */
35 int G_pole_in_polygon(const double *x, const double *y, int n)
36 {
37  int i;
38  double len, area, total_len, total_area;
39 
40  if (n <= 1)
41  return 0;
42 
43  mystats(x[n - 1], y[n - 1], x[0], y[0], &total_len, &total_area);
44  for (i = 1; i < n; i++) {
45  mystats(x[i - 1], y[i - 1], x[i], y[i], &len, &area);
46  total_len += len;
47  total_area += area;
48  }
49 
50  /* if polygon contains a pole then the x-coordinate length of
51  * the perimeter should compute to 0, otherwise it should be about 360
52  * (or -360, depending on the direction of perimeter traversal)
53  *
54  * instead of checking for exactly 0, check from -1 to 1 to avoid
55  * roundoff error.
56  */
57  if (total_len < 1.0 && total_len > -1.0)
58  return 0;
59 
60  return total_area >= 0.0 ? 1 : -1;
61 }
62 
63 static void mystats(double x0, double y0, double x1, double y1, double *len,
64  double *area)
65 {
66  if (x1 > x0)
67  while (x1 - x0 > 180)
68  x0 += 360;
69  else if (x0 > x1)
70  while (x0 - x1 > 180)
71  x0 -= 360;
72 
73  *len = x0 - x1;
74 
75  if (x0 > x1)
76  *area = (x0 - x1) * (y0 + y1) / 2.0;
77  else
78  *area = (x1 - x0) * (y1 + y0) / 2.0;
79 }
#define x
int G_pole_in_polygon(const double *x, const double *y, int n)
Check if pole is in polygon.
Definition: pole_in_poly.c:35
double y
Definition: dataquad.h:43