NAME¶
MagneticField -- compute the earth's magnetic field
SYNOPSIS¶
MagneticField [
-n name ] [
-d dir ] [
-t time |
-c time lat h ] [
-r ] [
-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 or a fractional year such as
2012.5.
lat and
lon are the latitude and longitude expressed as
decimal degrees or degrees, minutes, and seconds; see
GeoConvert(1) for
details.
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
"wmm2010". 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.
- -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
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
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
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 "wmm2010" 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 "wmm2010".
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 WMM2010 in Timbuktu on 2012-01-01
echo 2012-01-01 16:46:33N 3:00:34W 300 | MagneticField -r
=> -2.55 12.43 33771.0 33737.6 -1500.5 7446.0 34582.1
0.10 -0.07 34.3 36.8 54.4 -35.3 25.9
The first two numbers returned are the declination and inclination of the field.
The second line gives the annual change.
SEE ALSO¶
GeoConvert(1).
AUTHOR¶
MagneticField was written by Charles Karney.
HISTORY¶
MagneticField was added to GeographicLib,
<
http://geographiclib.sf.net>, in version 1.15.