GRASS 8 Programmer's Manual
8.6.0dev(2026)-f6f2c534ea
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pole_in_poly.c
Go to the documentation of this file.
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/*!
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\file lib/gis/pole_in_poly.c
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\brief GIS Library - Pole in polygon
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(C) 2001-2009 by the GRASS Development Team
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This program is free software under the GNU General Public License
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(>=v2). Read the file COPYING that comes with GRASS for details.
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\author CERL
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*/
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#include <
grass/gis.h
>
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static
void
mystats(
double
,
double
,
double
,
double
,
double
*,
double
*);
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/*!
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* \brief Check if pole is in polygon
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*
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* For latitude-longitude coordinates, this routine determines if the polygon
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* defined by the <i>n</i> coordinate vertices <i>x,y</i> contains one of the
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* poles.
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*
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* <b>Note:</b> Use this routine only if the projection is PROJECTION_LL.
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*
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* \param x array of x coordinates
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* \param y array of y coordinates
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* \param n number of coordinates
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*
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* \return -1 if it contains the south pole
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* \return 1 if it contains the north pole
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* \return 0 if it contains neither pole.
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*/
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int
G_pole_in_polygon
(
const
double
*x,
const
double
*y,
int
n)
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{
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int
i;
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double
len, area,
total_len
,
total_area
;
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if
(n <= 1)
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return
0;
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mystats(
x
[n - 1], y[n - 1],
x
[0], y[0], &
total_len
, &
total_area
);
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for
(i = 1; i < n; i++) {
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mystats(
x
[i - 1], y[i - 1],
x
[i], y[i], &len, &area);
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total_len
+= len;
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total_area
+= area;
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}
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/* if polygon contains a pole then the x-coordinate length of
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* the perimeter should compute to 0, otherwise it should be about 360
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* (or -360, depending on the direction of perimeter traversal)
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*
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* instead of checking for exactly 0, check from -1 to 1 to avoid
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* roundoff error.
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*/
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if
(
total_len < 1.0 && total_len >
-1.0)
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return
0;
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return
total_area
>= 0.0 ? 1 : -1;
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}
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static
void
mystats(
double
x0,
double
y0
,
double
x1,
double
y1,
double
*len,
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double
*area)
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{
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if
(x1 > x0)
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while
(x1 - x0 > 180)
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x0 += 360;
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else
if
(x0 > x1)
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while
(x0 - x1 > 180)
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x0 -= 360;
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*len = x0 - x1;
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if
(x0 > x1)
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*area = (x0 - x1) * (
y0
+ y1) / 2.0;
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else
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*area = (x1 - x0) * (y1 +
y0
) / 2.0;
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}
AMI_STREAM
Definition
ami_stream.h:153
gis.h
G_pole_in_polygon
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
x
#define x
lib
gis
pole_in_poly.c
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