GRASS GIS 8 Programmer's Manual 8.3.2(2024)-exported
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rhumbline.c
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1/*!
2 * \file lib/gis/rhumbline.c
3 *
4 * \brief GIS Library - Rhumbline calculation routines.
5 *
6 * From "Map Projections" by Peter Richardus and Ron K. Alder, 1972<br>
7 * (526.8 R39m in Map & Geography Library)<br>
8 * Page 20,21, formulas 2.21, 2.22
9 *
10 * Formula is the equation of a rhumbline from (lat1,lon1) to
11 * (lat2,lon2). Input is lon, output is lat (all in degrees).
12 *
13 * <b>Note:</b> Formula only works if 0 < abs(lon2-lon1) < 180.
14 * If lon1 == lon2 then rhumbline is the merdian lon1 (and the formula
15 * will fail).
16 * <br>
17 * <b>WARNING:</b> This code is preliminary. It may not even be correct.
18 *
19 * (C) 2001-2014 by the GRASS Development Team
20 *
21 * This program is free software under the GNU General Public License
22 * (>=v2). Read the file COPYING that comes with GRASS for details.
23 *
24 * \author GRASS GIS Development Team
25 *
26 * \date 1999-2014
27 */
28
29#include <math.h>
30#include <grass/gis.h>
31#include "pi.h"
32
33static void adjust_lat(double *);
34
35#if 0
36static void adjust_lon(double *);
37#endif /* unused */
38
39static struct state {
40 double TAN_A, TAN1, TAN2, L;
41 int parallel;
42} state;
43
44static struct state *st = &state;
45
46/**
47 * \brief Start rhumbline calculations.
48 *
49 * <b>Note:</b> This function must be called before other rhumbline
50 * functions to initialize parameters.
51 *
52 * \param[in] lon1,lat1 longitude, latitude of first point
53 * \param[in] lon2,lat2 longitude, latitude of second point
54 * \return 1 on success
55 * \return 0 on error
56 */
57
58int G_begin_rhumbline_equation(double lon1, double lat1, double lon2,
59 double lat2)
60{
61 adjust_lat(&lat1);
62 adjust_lat(&lat2);
63
64 if (lon1 == lon2) {
65 st->parallel = 1; /* a lie */
66 st->L = lat1;
67 return 0;
68 }
69 if (lat1 == lat2) {
70 st->parallel = 1;
71 st->L = lat1;
72 return 1;
73 }
74 st->parallel = 0;
75 lon1 = Radians(lon1);
76 lon2 = Radians(lon2);
77 lat1 = Radians(lat1);
78 lat2 = Radians(lat2);
79
80 st->TAN1 = tan(M_PI_4 + lat1 / 2.0);
81 st->TAN2 = tan(M_PI_4 + lat2 / 2.0);
82 st->TAN_A = (lon2 - lon1) / (log(st->TAN2) - log(st->TAN1));
83 st->L = lon1;
84
85 return 1;
86}
87
88/**
89 * \brief Calculates rhumbline latitude.
90 *
91 * <b>Note:</b> Function only works if lon1 < lon < lon2.
92 *
93 * \param[in] lon longitude
94 * \return double latitude in degrees
95 */
96
97double G_rhumbline_lat_from_lon(double lon)
98{
99 if (st->parallel)
100 return st->L;
101
102 lon = Radians(lon);
103
104 return Degrees(2 * atan(exp((lon - st->L) / st->TAN_A) * st->TAN1) -
105 M_PI_2);
106}
107
108#if 0
109static void adjust_lon(double *lon)
110{
111 while (*lon > 180.0)
112 *lon -= 360.0;
113 while (*lon < -180.0)
114 *lon += 360.0;
115}
116#endif /* unused */
117
118static void adjust_lat(double *lat)
119{
120 if (*lat > 90.0)
121 *lat = 90.0;
122 if (*lat < -90.0)
123 *lat = -90.0;
124}
struct state state
Definition parser.c:103
struct state * st
Definition parser.c:104
#define Degrees(x)
Definition pi.h:7
#define Radians(x)
Definition pi.h:6
int G_begin_rhumbline_equation(double lon1, double lat1, double lon2, double lat2)
Start rhumbline calculations.
Definition rhumbline.c:58
double G_rhumbline_lat_from_lon(double lon)
Calculates rhumbline latitude.
Definition rhumbline.c:97