1=head1 NAME
2
3MagneticField -- compute the earth's magnetic field
4
5=head1 SYNOPSIS
6
7B<MagneticField> [ B<-n> I<name> ] [ B<-d> I<dir> ]
8[ B<-N> I<Nmax> ] [ B<-M> I<Mmax> ]
9[ B<-t> I<time> | B<-c> I<time> I<lat> I<h> ]
10[ B<-r> ] [ B<-w> ] [ B<-T> I<tguard> ] [ B<-H> I<hguard> ] [ B<-p> I<prec> ]
11[ B<-v> ]
12[ B<--comment-delimiter> I<commentdelim> ]
13[ B<--version> | B<-h> | B<--help> ]
14[ B<--input-file> I<infile> | B<--input-string> I<instring> ]
15[ B<--line-separator> I<linesep> ]
16[ B<--output-file> I<outfile> ]
17
18=head1 DESCRIPTION
19
20B<MagneticField> reads in times and positions on standard input and
21prints out the geomagnetic field on standard output and, optionally, its
22rate of change.
23
24The input line is of the form I<time> I<lat> I<lon> I<h>. I<time> is a
25date of the form 2012-07-03, a fractional year such as 2012.5, or the
26string "now".  I<lat> and I<lon> are the latitude and longitude
27expressed as decimal degrees or degrees, minutes, and seconds; for
28details on the allowed formats for latitude and longitude, see the
29C<GEOGRAPHIC COORDINATES> section of GeoConvert(1).  I<h> is the height
30above the ellipsoid in meters; this is optional and defaults to zero.
31Alternatively, I<time> can be given on the command line as the argument
32to the B<-t> option, in which case it should not be included on the
33input lines.  Finally, the magnetic field can be computed at various
34points on a circle of latitude (constant I<time>, I<lat>, and I<h>) via
35the B<-c> option; in this case only the longitude should be given on the
36input lines.
37
38The output consists of the following 7 items:
39
40  the declination (the direction of the horizontal component of
41    the magnetic field measured clockwise from north) in degrees,
42  the inclination (the direction of the magnetic field measured
43    down from the horizontal) in degrees,
44  the horizontal field in nanotesla (nT),
45  the north component of the field in nT,
46  the east component of the field in nT,
47  the vertical component of the field in nT (down is positive),
48  the total field in nT.
49
50If the B<-r> option is given, a second line is printed giving the rates
51of change of these quantities in degrees/yr and nT/yr.
52
53The WGS84 ellipsoid is used, I<a> = 6378137 m, I<f> = 1/298.257223563.
54
55=head1 OPTIONS
56
57=over
58
59=item B<-n> I<name>
60
61use magnetic field model I<name> instead of the default C<wmm2020>.  See
62L</MODELS>.
63
64=item B<-d> I<dir>
65
66read magnetic models from I<dir> instead of the default.  See
67L</MODELS>.
68
69=item B<-N> I<Nmax>
70
71limit the degree of the model to I<Nmax>.
72
73=item B<-M> I<Mmax>
74
75limit the order of the model to I<Mmax>.
76
77=item B<-t> I<time>
78
79evaluate the field at I<time> instead of reading the time from the input
80lines.
81
82=item B<-c> I<time> I<lat> I<h>
83
84evaluate the field on a circle of latitude given by I<time>, I<lat>,
85I<h> instead of reading these quantities from the input lines.  In this
86case, B<MagneticField> can calculate the field considerably more
87quickly.
88
89=item B<-r>
90
91toggle whether to report the rates of change of the field.
92
93=item B<-w>
94
95toggle the longitude first flag (it starts off); if the flag is on, then
96on input and output, longitude precedes latitude (except that, on input,
97this can be overridden by a hemisphere designator, I<N>, I<S>, I<E>,
98I<W>).
99
100=item B<-T> I<tguard>
101
102signal an error if I<time> lies I<tguard> years (default 50 yr) beyond
103the range for the model.
104
105=item B<-H> I<hguard>
106
107signal an error if I<h> lies I<hguard> meters (default 500000 m) beyond
108the range for the model.
109
110=item B<-p> I<prec>
111
112set the output precision to I<prec> (default 1).  Fields are printed
113with precision with I<prec> decimal places; angles use I<prec> + 1
114places.
115
116=item B<-v>
117
118print information about the magnetic model on standard error before
119processing the input.
120
121=item B<--comment-delimiter> I<commentdelim>
122
123set the comment delimiter to I<commentdelim> (e.g., "#" or "//").  If
124set, the input lines will be scanned for this delimiter and, if found,
125the delimiter and the rest of the line will be removed prior to
126processing and subsequently appended to the output line (separated by a
127space).
128
129=item B<--version>
130
131print version and exit.
132
133=item B<-h>
134
135print usage, the default magnetic path and name, and exit.
136
137=item B<--help>
138
139print full documentation and exit.
140
141=item B<--input-file> I<infile>
142
143read input from the file I<infile> instead of from standard input; a file
144name of "-" stands for standard input.
145
146=item B<--input-string> I<instring>
147
148read input from the string I<instring> instead of from standard input.
149All occurrences of the line separator character (default is a semicolon)
150in I<instring> are converted to newlines before the reading begins.
151
152=item B<--line-separator> I<linesep>
153
154set the line separator character to I<linesep>.  By default this is a
155semicolon.
156
157=item B<--output-file> I<outfile>
158
159write output to the file I<outfile> instead of to standard output; a
160file name of "-" stands for standard output.
161
162=back
163
164=head1 MODELS
165
166B<MagneticField> computes the geomagnetic field using one of the
167following models
168
169    wmm2010, the World Magnetic Model 2010, which approximates the
170      main magnetic field for the period 2010-2015.  See
171      https://ngdc.noaa.gov/geomag/WMM/DoDWMM.shtml
172    wmm2015v2, the World Magnetic Model 2015, which approximates the
173      main magnetic field for the period 2015-2020.  See
174      https://ngdc.noaa.gov/geomag/WMM/DoDWMM.shtml
175    wmm2015, a deprecated version of wmm2015v2
176    wmm2020, the World Magnetic Model 2020, which approximates the
177      main magnetic field for the period 2020-2025.  See
178      https://ngdc.noaa.gov/geomag/WMM/DoDWMM.shtml
179    igrf11, the International Geomagnetic Reference Field (11th
180      generation), which approximates the main magnetic field for
181      the period 1900-2015.  See
182      https://ngdc.noaa.gov/IAGA/vmod/igrf.html
183    igrf12, the International Geomagnetic Reference Field (12th
184      generation), which approximates the main magnetic field for
185      the period 1900-2020.  See
186      https://ngdc.noaa.gov/IAGA/vmod/igrf.html
187    igrf13, the International Geomagnetic Reference Field (13th
188      generation), which approximates the main magnetic field for
189      the period 1900-2025.  See
190      https://ngdc.noaa.gov/IAGA/vmod/igrf.html
191    emm2010, the Enhanced Magnetic Model 2010, which approximates
192      the main and crustal magnetic fields for the period 2010-2015.
193      See https://ngdc.noaa.gov/geomag/EMM/index.html
194    emm2015, the Enhanced Magnetic Model 2015, which approximates
195      the main and crustal magnetic fields for the period 2000-2020.
196      See https://ngdc.noaa.gov/geomag/EMM/index.html
197    emm2017, the Enhanced Magnetic Model 2017, which approximates
198      the main and crustal magnetic fields for the period 2000-2022.
199      See https://ngdc.noaa.gov/geomag/EMM/index.html
200
201These models approximate the magnetic field due to the earth's core and
202(in the case of emm20xx) its crust.  They neglect magnetic fields due to
203the ionosphere, the magnetosphere, nearby magnetized materials,
204electrical machinery, etc.
205
206By default, the C<wmm2020> magnetic model is used.  This may changed by
207setting the environment variable C<GEOGRAPHICLIB_MAGNETIC_NAME> or with
208the B<-n> option.
209
210The magnetic models will be loaded from a directory specified at compile
211time.  This may changed by setting the environment variables
212C<GEOGRAPHICLIB_MAGNETIC_PATH> or C<GEOGRAPHICLIB_DATA>, or with the
213B<-d> option.  The B<-h> option prints the default magnetic path and
214name.  Use the B<-v> option to ascertain the full path name of the data
215file.
216
217Instructions for downloading and installing magnetic models are
218available at
219L<https://geographiclib.sourceforge.io/html/magnetic.html#magneticinst>.
220
221=head1 ENVIRONMENT
222
223=over
224
225=item B<GEOGRAPHICLIB_MAGNETIC_NAME>
226
227Override the compile-time default magnetic name of C<wmm2020>.  The
228B<-h> option reports the value of B<GEOGRAPHICLIB_MAGNETIC_NAME>, if
229defined, otherwise it reports the compile-time value.  If the B<-n>
230I<name> option is used, then I<name> takes precedence.
231
232=item B<GEOGRAPHICLIB_MAGNETIC_PATH>
233
234Override the compile-time default magnetic path.  This is typically
235C</usr/local/share/GeographicLib/magnetic> on Unix-like systems and
236C<C:/ProgramData/GeographicLib/magnetic> on Windows systems.  The B<-h>
237option reports the value of B<GEOGRAPHICLIB_MAGNETIC_PATH>, if defined,
238otherwise it reports the compile-time value.  If the B<-d> I<dir> option
239is used, then I<dir> takes precedence.
240
241=item B<GEOGRAPHICLIB_DATA>
242
243Another way of overriding the compile-time default magnetic path.  If it
244is set (and if B<GEOGRAPHICLIB_MAGNETIC_PATH> is not set), then
245$B<GEOGRAPHICLIB_DATA>/magnetic is used.
246
247=back
248
249=head1 ERRORS
250
251An illegal line of input will print an error message to standard output
252beginning with C<ERROR:> and causes B<MagneticField> to return an exit
253code of 1.  However, an error does not cause B<MagneticField> to
254terminate; following lines will be converted.  If I<time> or I<h> are
255outside the recommended ranges for the model (but inside the ranges
256increase by I<tguard> and I<hguard>), a warning is printed on standard
257error and the field (which may be inaccurate) is returned in the normal
258way.
259
260=head1 EXAMPLES
261
262The magnetic field from WMM2020 in Timbuktu on 2020-01-01
263
264    echo 2020-01-01 16:46:33N 3:00:34W 300 | MagneticField -r
265    => -1.60 12.00 33973.5 33960.3 -948.1 7223.0 34732.8
266       0.13 -0.02 21.8 23.9 77.9 -8.4 19.5
267
268The first two numbers returned are the declination and inclination of
269the field.  The second line gives the annual change.
270
271=head1 SEE ALSO
272
273GeoConvert(1), geographiclib-get-magnetic(8).
274
275=head1 AUTHOR
276
277B<MagneticField> was written by Charles Karney.
278
279=head1 HISTORY
280
281B<MagneticField> was added to GeographicLib,
282L<https://geographiclib.sourceforge.io>, in version 1.15.
283