GRASS 8 Programmer's Manual
8.6.0dev(2026)-f6f2c534ea
Loading...
Searching...
No Matches
rhumbline.c
Go to the documentation of this file.
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 Development Team
25
*
26
* \date 1999-2014
27
*/
28
29
#include <math.h>
30
#include <
grass/gis.h
>
31
#include "
pi.h
"
32
33
static
void
adjust_lat(
double
*);
34
35
#if 0
36
static
void
adjust_lon(
double
*);
37
#endif
/* unused */
38
39
static
struct
state
{
40
double
TAN_A, TAN1, TAN2, L;
41
int
parallel;
42
}
state
;
43
44
static
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
int
G_begin_rhumbline_equation
(
double
lon1
,
double
lat1
,
double
lon2
,
58
double
lat2
)
59
{
60
adjust_lat(&
lat1
);
61
adjust_lat(&
lat2
);
62
63
if
(
lon1
==
lon2
) {
64
st
->parallel = 1;
/* a lie */
65
st
->L =
lat1
;
66
return
0;
67
}
68
if
(
lat1
==
lat2
) {
69
st
->parallel = 1;
70
st
->L =
lat1
;
71
return
1;
72
}
73
st
->parallel = 0;
74
lon1
=
Radians
(
lon1
);
75
lon2
=
Radians
(
lon2
);
76
lat1
=
Radians
(
lat1
);
77
lat2
=
Radians
(
lat2
);
78
79
st
->TAN1 =
tan
(
M_PI_4
+
lat1
/ 2.0);
80
st
->TAN2 =
tan
(
M_PI_4
+
lat2
/ 2.0);
81
st
->TAN_A = (
lon2
-
lon1
) / (
log
(
st
->TAN2) -
log
(
st
->TAN1));
82
st
->L =
lon1
;
83
84
return
1;
85
}
86
87
/**
88
* \brief Calculates rhumbline latitude.
89
*
90
* <b>Note:</b> Function only works if lon1 < lon < lon2.
91
*
92
* \param[in] lon longitude
93
* \return double latitude in degrees
94
*/
95
double
G_rhumbline_lat_from_lon
(
double
lon
)
96
{
97
if
(
st
->parallel)
98
return
st
->L;
99
100
lon
=
Radians
(
lon
);
101
102
return
Degrees
(2 *
atan
(
exp
((
lon
-
st
->L) /
st
->TAN_A) *
st
->TAN1) -
103
M_PI_2
);
104
}
105
106
#if 0
107
static
void
adjust_lon(
double
*
lon
)
108
{
109
while
(*
lon
> 180.0)
110
*
lon
-= 360.0;
111
while
(*
lon
< -180.0)
112
*
lon
+= 360.0;
113
}
114
#endif
/* unused */
115
116
static
void
adjust_lat(
double
*
lat
)
117
{
118
if
(*
lat
> 90.0)
119
*
lat
= 90.0;
120
if
(*
lat
< -90.0)
121
*
lat
= -90.0;
122
}
AMI_STREAM
Definition
ami_stream.h:153
gis.h
M_PI_2
#define M_PI_2
Definition
gis.h:160
M_PI_4
#define M_PI_4
Definition
gis.h:168
state
struct state state
Definition
parser.c:103
st
struct state * st
Definition
parser.c:104
pi.h
Degrees
#define Degrees(x)
Definition
pi.h:7
Radians
#define Radians(x)
Definition
pi.h:6
G_begin_rhumbline_equation
int G_begin_rhumbline_equation(double lon1, double lat1, double lon2, double lat2)
Start rhumbline calculations.
Definition
rhumbline.c:57
G_rhumbline_lat_from_lon
double G_rhumbline_lat_from_lon(double lon)
Calculates rhumbline latitude.
Definition
rhumbline.c:95
lib
gis
rhumbline.c
Generated on Sun Apr 12 2026 06:59:15 for GRASS 8 Programmer's Manual by
1.9.8