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PROGRAM:

NAME


MagneticField -- compute the earth's magnetic field

SYNOPSIS


MagneticField [ -n name ] [ -d dir ] [ -t time | -c time lat h ] [ -r ] [ -w ] [ -T tguard
] [ -H hguard ] [ -p prec ] [ -v ] [ --comment-delimiter commentdelim ] [ --version | -h |
--help ] [ --input-file infile | --input-string instring ] [ --line-separator linesep ] [
--output-file outfile ]

DESCRIPTION


MagneticField reads in times and positions on standard input and prints out the
geomagnetic field on standard output and, optionally, its rate of change.

The input line is of the form time lat lon h. time is a date of the form 2012-07-03, a
fractional year such as 2012.5, or the string "now". lat and lon are the latitude and
longitude expressed as decimal degrees or degrees, minutes, and seconds; for details on
the allowed formats for latitude and longitude, see the "GEOGRAPHIC COORDINATES" section
of GeoConvert(1). h is the height above the ellipsoid in meters; this is optional and
defaults to zero. Alternatively, time can be given on the command line as the argument to
the -t option, in which case it should not be included on the input lines. Finally, the
magnetic field can be computed at various points on a circle of latitude (constant time,
lat, and h) via the -c option; in this case only the longitude should be given on the
input lines.

The output consists of the following 7 items:

the declination (the direction of the horizontal component of
the magnetic field measured clockwise from north) in degrees,
the inclination (the direction of the magnetic field measured
down from the horizontal) in degrees,
the horizontal field in nanotesla (nT),
the north component of the field in nT,
the east component of the field in nT,
the vertical component of the field in nT (down is positive),
the total field in nT.

If the -r option is given, a second line is printed giving the rates of change of these
quantities in degrees/yr and nT/yr.

The WGS84 ellipsoid is used, a = 6378137 m, f = 1/298.257223563.

OPTIONS


-n use magnetic field model name instead of the default "wmm2015". See "MODELS".

-d read magnetic models from dir instead of the default. See "MODELS".

-t evaluate the field at time instead of reading the time from the input lines.

-c evaluate the field on a circle of latitude given by time, lat, h instead of reading
these quantities from the input lines. In this case, MagneticField can calculate the
field considerably more quickly.

-r toggle whether to report the rates of change of the field.

-w on input and output, longitude precedes latitude (except that on input this can be
overridden by a hemisphere designator, N, S, E, W).

-T signal an error if time lies tguard years (default 50 yr) beyond the range for the
model.

-H signal an error if h lies hguard meters (default 500000 m) beyond the range for the
model.

-p set the output precision to prec (default 1). Fields are printed with precision with
prec decimal places; angles use prec + 1 places.

-v print information about the magnetic model on standard error before processing the
input.

--comment-delimiter
set the comment delimiter to commentdelim (e.g., "#" or "//"). If set, the input
lines will be scanned for this delimiter and, if found, the delimiter and the rest of
the line will be removed prior to processing and subsequently appended to the output
line (separated by a space).

--version
print version and exit.

-h print usage, the default magnetic path and name, and exit.

--help
print full documentation and exit.

--input-file
read input from the file infile instead of from standard input; a file name of "-"
stands for standard input.

--input-string
read input from the string instring instead of from standard input. All occurrences
of the line separator character (default is a semicolon) in instring are converted to
newlines before the reading begins.

--line-separator
set the line separator character to linesep. By default this is a semicolon.

--output-file
write output to the file outfile instead of to standard output; a file name of "-"
stands for standard output.

MODELS


MagneticField computes the geomagnetic field using one of the following models

wmm2010, the World Magnetic Model 2010, which approximates the
main magnetic field for the period 2010-2015. See
http://ngdc.noaa.gov/geomag/WMM/DoDWMM.shtml
wmm2015, the World Magnetic Model 2015, which approximates the
main magnetic field for the period 2015-2020. See
http://ngdc.noaa.gov/geomag/WMM/DoDWMM.shtml
igrf11, the International Geomagnetic Reference Field (11th
generation), which approximates the main magnetic field for
the period 1900-2015. See
http://ngdc.noaa.gov/IAGA/vmod/igrf.html
igrf12, the International Geomagnetic Reference Field (12th
generation), which approximates the main magnetic field for
the period 1900-2020. See
http://ngdc.noaa.gov/IAGA/vmod/igrf.html
emm2010, the Enhanced Magnetic Model 2010, which approximates
the main and crustal magnetic fields for the period 2010-2015.
See http://ngdc.noaa.gov/geomag/EMM/index.html
emm2015, the Enhanced Magnetic Model 2015, which approximates
the main and crustal magnetic fields for the period 2000-2020.
See http://ngdc.noaa.gov/geomag/EMM/index.html

These models approximate the magnetic field due to the earth's core and (in the case of
emm2010) its crust. They neglect magnetic fields due to the ionosphere, the
magnetosphere, nearby magnetized materials, electrical machinery, etc.

By default, the "wmm2015" magnetic model is used. This may changed by setting the
environment variable "GEOGRAPHICLIB_MAGNETIC_NAME" or with the -n option.

The magnetic models will be loaded from a directory specified at compile time. This may
changed by setting the environment variables "GEOGRAPHICLIB_MAGNETIC_PATH" or
"GEOGRAPHICLIB_DATA", or with the -d option. The -h option prints the default magnetic
path and name. Use the -v option to ascertain the full path name of the data file.

Instructions for downloading and installing magnetic models are available at
<http://geographiclib.sf.net/html/magnetic.html#magneticinst>.

ENVIRONMENT


GEOGRAPHICLIB_MAGNETIC_NAME
Override the compile-time default magnetic name of "wmm2015". The -h option reports
the value of GEOGRAPHICLIB_MAGNETIC_NAME, if defined, otherwise it reports the
compile-time value. If the -n name option is used, then name takes precedence.

GEOGRAPHICLIB_MAGNETIC_PATH
Override the compile-time default magnetic path. This is typically
"/usr/local/share/GeographicLib/magnetic" on Unix-like systems and
"C:/ProgramData/GeographicLib/magnetic" on Windows systems. The -h option reports the
value of GEOGRAPHICLIB_MAGNETIC_PATH, if defined, otherwise it reports the compile-
time value. If the -d dir option is used, then dir takes precedence.

GEOGRAPHICLIB_DATA
Another way of overriding the compile-time default magnetic path. If it is set (and
if GEOGRAPHICLIB_MAGNETIC_PATH is not set), then $GEOGRAPHICLIB_DATA/magnetic is used.

ERRORS


An illegal line of input will print an error message to standard output beginning with
"ERROR:" and causes MagneticField to return an exit code of 1. However, an error does not
cause MagneticField to terminate; following lines will be converted. If time or h are
outside the recommended ranges for the model (but inside the ranges increase by tguard and
hguard), a warning is printed on standard error and the field (which may be inaccurate) is
returned in the normal way.

EXAMPLES


The magnetic field from WMM2015 in Timbuktu on 2016-01-01

echo 2016-01-01 16:46:33N 3:00:34W 300 | MagneticField -r
=> -2.12 12.15 33871.9 33848.7 -1251.4 7293.9 34648.3
0.09 -0.08 31.8 33.8 53.7 -41.4 22.3

The first two numbers returned are the declination and inclination of the field. The
second line gives the annual change.

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