GeographicLib can compute the earth's magnetic field by a magnetic model using the GeographicLib::MagneticModel and GeographicLib::MagneticCircle classes and with the MagneticField utility. These models expand the internal magnetic potential of the earth as sum of spherical harmonics. They neglect magnetic fields due to the ionosphere, the magnetosphere, nearby magnetized materials, electric machinery, etc. Users of GeographicLib::MagneticModel are advised to read the "Health Warning" this is provided with igrf11. Although the advice is specific to igrf11, many of the comments apply to all magnetic field models.
The supported models are
These magnetic models are available for download:
The "size" column is the size of the uncompressed data.
For Linux and Unix systems, GeographicLib provides a shell script geographiclib-get-magnetic (typically installed in /usr/local/sbin) which automates the process of downloading and installing the magnetic models. For example
geographiclib-get-magnetic all # to install wmm2010, igrf11, emm2010 geographiclib-get-magnetic -h # for help
This script should be run as a user with write access to the installation directory, which is typically /usr/local/share/GeographicLib (this can be overridden with the -p flag), and the data will then be placed in the "magnetic" subdirectory.
Windows users should download and run the Windows installers. These will prompt for an installation directory with the default being one of
C:/Documents and Settings/All Users/Application Data/GeographicLib C:/ProgramData/GeographicLib
(which you probably should not change) and the data is installed in the "magnetic" sub-directory. (The second directory name is an alternate name that Windows 7 for the "Application Data" directory.)
Otherwise download either the tar.bz2 file or the zip file (they have the same contents). To unpack these, run, for example
mkdir -p /usr/local/share/GeographicLib tar xofjC wmm2010.tar.bz2 /usr/local/share/GeographicLib tar xofjC emm2010.tar.bz2 /usr/local/share/GeographicLib etc.
and, again, the data will be placed in the "magnetic" subdirectory.
However you install the magnetic models, all the datasets should be installed in the same directory. GeographicLib::MagneticModel and MagneticField uses a compile time default to locate the datasets. This is
consistent with the examples above. This may be overridden at run-time by defining the MAGNETIC_PATH or the GEOGRAPHIC_DATA environment variables; see GeographicLib::MagneticModel::DefaultMagneticPath() for details. Finally, the path may be set using the optional second argument to the GeographicLib::MagneticModel constructor or with the "-d" flag to MagneticField. Supplying the "-h" flag to MagneticField reports the default path for magnetic models for that utility. The "-v" flag causes MagneticField to report the full path name of the data file it uses.
The constructor for GeographicLib::MagneticModel reads a file called NAME.wmm which specifies various properties for the magnetic model. It then opens a binary file NAME.wmm.cof to obtain the coefficients of the spherical harmonic sum.
The first line of the .wmm file must consist of "WMMF-v" where WMMF stands for "World Magnetic Model Format" and v is the version number of the format (currently "1").
The rest of the File is read a line at a time. A # character and everything after it are discarded. If the result is just white space it is discarded. The remaining lines are of the form "KEY WHITESPACE VALUE". In general, the KEY and the VALUE are case-sensitive.
GeographicLib::MagneticModel only pays attention to the following keywords
Other keywords are ignored.
The coefficient file NAME.wmm.cof is a binary file in little endian order. The first 8 bytes of this file must match the ID given in NAME.wmm. This is followed by NumModels + 1 sets of spherical harmonic coefficients. The first NumModels of these model the magnetic field at Epoch + i * DeltaEpoch for 0 ≤ i < NumModels. The last set of coefficients model the rate of change of the magnetic field at Epoch + (NumModels − 1) * DeltaEpoch. The format for each set of coefficients is:
Although the coefficient file is in little endian order, GeographicLib can read it on big endian machines. It can only be read on machines which store doubles in IEEE format.
As an illustration, here is igrf11.wmm:
WMMF-1 # A World Magnetic Model (Format 1) file. For documentation on the # format of this file see # http://geographiclib.sf.net/html/magnetic.html#magneticformat Name igrf11 Description International Geomagnetic Reference Field 11th Generation URL http://ngdc.noaa.gov/IAGA/vmod/igrf.html Publisher National Oceanic and Atmospheric Administration ReleaseDate 2009-12-15 DataCutOff 2009-10-01 ConversionDate 2011-11-04 DataVersion 1 Radius 6371200 NumModels 23 Epoch 1900 DeltaEpoch 5 MinTime 1900 MaxTime 2015 MinHeight -1000 MaxHeight 600000 # The coefficients are stored in a file obtained by appending ".cof" to # the name of this file. The coefficients were obtained from IGRF11.COF # in the geomag70 distribution. ID IGRF11-A