GeographicLib::MagneticModel Class Reference

Model of the earth's magnetic field. More...

#include <GeographicLib/MagneticModel.hpp>

Public Member Functions
Setting up the magnetic model
	MagneticModel (const std::string &name, const std::string &path="", const Geocentric &earth=Geocentric::WGS84())
Inspector functions
const std::string &	Description () const
const std::string &	DateTime () const
const std::string &	MagneticFile () const
const std::string &	MagneticModelName () const
const std::string &	MagneticModelDirectory () const
Math::real	MinHeight () const
Math::real	MaxHeight () const
Math::real	MinTime () const
Math::real	MaxTime () const
Math::real	MajorRadius () const
Math::real	Flattening () const

Static Public Member Functions
static std::string	DefaultMagneticPath ()
static std::string	DefaultMagneticName ()

Compute the magnetic field
void	operator() (real t, real lat, real lon, real h, real &Bx, real &By, real &Bz) const
void	operator() (real t, real lat, real lon, real h, real &Bx, real &By, real &Bz, real &Bxt, real &Byt, real &Bzt) const
MagneticCircle	Circle (real t, real lat, real h) const
static void	FieldComponents (real Bx, real By, real Bz, real &H, real &F, real &D, real &I)
static void	FieldComponents (real Bx, real By, real Bz, real Bxt, real Byt, real Bzt, real &H, real &F, real &D, real &I, real &Ht, real &Ft, real &Dt, real &It)

Detailed Description

Model of the earth's magnetic field.

Evaluate the earth's magnetic field according to a model. At present only internal magnetic fields are handled. These are due to the earth's code and crust; these vary slowly (over many years). Excluded are the effects of currents in the ionosphere and magnetosphere which have daily and annual variations.

See Magnetic models for details of how to install the magnetic models and the data format.

See

• General information:
  • http://geomag.org/models/index.html
• WMM2010:
  • http://ngdc.noaa.gov/geomag/WMM/DoDWMM.shtml
  • http://ngdc.noaa.gov/geomag/WMM/data/WMM2010/WMM2010COF.zip
• WMM2015:
  • http://ngdc.noaa.gov/geomag/WMM/DoDWMM.shtml
  • http://ngdc.noaa.gov/geomag/WMM/data/WMM2015/WMM2015COF.zip
• IGRF11:
  • http://ngdc.noaa.gov/IAGA/vmod/igrf.html
  • http://ngdc.noaa.gov/IAGA/vmod/igrf11coeffs.txt
  • http://ngdc.noaa.gov/IAGA/vmod/geomag70_linux.tar.gz
• EMM2010:
  • http://ngdc.noaa.gov/geomag/EMM/index.html
  • http://ngdc.noaa.gov/geomag/EMM/data/geomag/EMM2010_Sph_Windows_Linux.zip

Example of use:

// Example of using the GeographicLib::MagneticModel class

#include <iostream>
#include <exception>
#include <GeographicLib/MagneticModel.hpp>

using namespace std;
using namespace GeographicLib;

int main() {
  try {
    MagneticModel mag("wmm2010");
    double lat = 27.99, lon = 86.93, h = 8820, t = 2012; // Mt Everest
    double Bx, By, Bz;
    mag(t, lat,lon, h, Bx, By, Bz);
    double H, F, D, I;
    MagneticModel::FieldComponents(Bx, By, Bz, H, F, D, I);
    cout << H << " " << F << " " << D << " " << I << "\n";
  }
  catch (const exception& e) {
    cerr << "Caught exception: " << e.what() << "\n";
    return 1;
  }
  return 0;
}

MagneticField is a command-line utility providing access to the functionality of MagneticModel and MagneticCircle.

Definition at line 63 of file MagneticModel.hpp.

Constructor & Destructor Documentation

GeographicLib::MagneticModel::MagneticModel ( const std::string &  name, const std::string &  path = "", const Geocentric &  earth = Geocentric::WGS84()  )

explicit

Construct a magnetic model.

Parameters

[in]	name	the name of the model.
[in]	path	(optional) directory for data file.
[in]	earth	(optional) Geocentric object for converting coordinates; default Geocentric::WGS84().

Exceptions

GeographicErr	if the data file cannot be found, is unreadable, or is corrupt.
std::bad_alloc	if the memory necessary for storing the model can't be allocated.

A filename is formed by appending ".wmm" (World Magnetic Model) to the name. If path is specified (and is non-empty), then the file is loaded from directory, path. Otherwise the path is given by the DefaultMagneticPath().

This file contains the metadata which specifies the properties of the model. The coefficients for the spherical harmonic sums are obtained from a file obtained by appending ".cof" to metadata file (so the filename ends in ".wwm.cof").

The model is not tied to a particular ellipsoidal model of the earth. The final earth argument to the constructor specifies an ellipsoid to allow geodetic coordinates to the transformed into the spherical coordinates used in the spherical harmonic sum.

Definition at line 37 of file MagneticModel.cpp.

References DefaultMagneticPath(), GeographicLib::SphericalHarmonic::FULL, GeographicLib::Math::isfinite(), GeographicLib::Utility::ParseLine(), GeographicLib::SphericalEngine::coeff::readcoeffs(), and GeographicLib::SphericalHarmonic::SCHMIDT.

Member Function Documentation

void GeographicLib::MagneticModel::operator() ( real  t, real  lat, real  lon, real  h, real &  Bx, real &  By, real &  Bz  ) const

inline

Evaluate the components of the geomagnetic field.

Parameters

[in]	t	the time (years).
[in]	lat	latitude of the point (degrees).
[in]	lon	longitude of the point (degrees).
[in]	h	the height of the point above the ellipsoid (meters).
[out]	Bx	the easterly component of the magnetic field (nanotesla).
[out]	By	the northerly component of the magnetic field (nanotesla).
[out]	Bz	the vertical (up) component of the magnetic field (nanotesla).

Definition at line 133 of file MagneticModel.hpp.

void GeographicLib::MagneticModel::operator() ( real  t, real  lat, real  lon, real  h, real &  Bx, real &  By, real &  Bz, real &  Bxt, real &  Byt, real &  Bzt  ) const

inline

Evaluate the components of the geomagnetic field and their time derivatives

Parameters

[in]	t	the time (years).
[in]	lat	latitude of the point (degrees).
[in]	lon	longitude of the point (degrees).
[in]	h	the height of the point above the ellipsoid (meters).
[out]	Bx	the easterly component of the magnetic field (nanotesla).
[out]	By	the northerly component of the magnetic field (nanotesla).
[out]	Bz	the vertical (up) component of the magnetic field (nanotesla).
[out]	Bxt	the rate of change of Bx (nT/yr).
[out]	Byt	the rate of change of By (nT/yr).
[out]	Bzt	the rate of change of Bz (nT/yr).

Definition at line 155 of file MagneticModel.hpp.

MagneticCircle GeographicLib::MagneticModel::Circle	(	real	t,
		real	lat,
		real	h
	)		const

Create a MagneticCircle object to allow the geomagnetic field at many points with constant lat, h, and t and varying lon to be computed efficiently.

Parameters

[in]	t	the time (years).
[in]	lat	latitude of the point (degrees).
[in]	h	the height of the point above the ellipsoid (meters).

Exceptions

std::bad_alloc

if the memory necessary for creating a MagneticCircle can't be allocated.

Returns

a MagneticCircle object whose MagneticCircle::operator()(real lon) member function computes the field at particular values of lon.

If the field at several points on a circle of latitude need to be calculated then creating a MagneticCircle and using its member functions will be substantially faster, especially for high-degree models.

Definition at line 212 of file MagneticModel.cpp.

Referenced by main().

static void GeographicLib::MagneticModel::FieldComponents ( real  Bx, real  By, real  Bz, real &  H, real &  F, real &  D, real &  I  )

inlinestatic

Compute various quantities dependent on the magnetic field.

Parameters

[in]	Bx	the x (easterly) component of the magnetic field (nT).
[in]	By	the y (northerly) component of the magnetic field (nT).
[in]	Bz	the z (vertical, up positive) component of t