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 throw ()
const std::string &	DateTime () const throw ()
const std::string &	MagneticFile () const throw ()
const std::string &	MagneticModelName () const throw ()
const std::string &	MagneticModelDirectory () const throw ()
Math::real	MinHeight () const throw ()
Math::real	MaxHeight () const throw ()
Math::real	MinTime () const throw ()
Math::real	MaxTime () const throw ()
Math::real	MajorRadius () const throw ()
Math::real	Flattening () const throw ()
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 throw ()
void	operator() (real t, real lat, real lon, real h, real &Bx, real &By, real &Bz, real &Bxt, real &Byt, real &Bzt) const throw ()
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) throw ()
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) throw ()

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 model and the data format.

See

Example of use:

// Example of using the GeographicLib::MagneticModel class
// $Id: ab27b155755e540d29e5e6bf550df8cdb00c74ba $

#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.

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.

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 may throw an exception because the file does not exist, is unreadable, or is corrupt.

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 specify an ellipsoid to allow geodetic coordinates to the transformed into the spherical coordinates used in the spherical harmonic sum.

Definition at line 44 of file MagneticModel.cpp.

References DefaultMagneticPath(), and GeographicLib::SphericalEngine::coeff::readcoeffs().

Member Function Documentation

void GeographicLib::MagneticModel::operator()	(	real	t,
		real	lat,
		real	lon,
		real	h,
		real &	Bx,
		real &	By,
		real &	Bz
	)		const throw () `[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 131 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 throw () `[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 153 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).

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 207 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
	)		throw () `[inline, static]`

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 the magnetic field (nT).
[out]	H	the horizontal magnetic field (nT).
[out]	F	the total magnetic field (nT).
[out]	D	the declination of the field (degrees east of north).
[out]	I	the inclination of the field (degrees down from horizontal).

Definition at line 190 of file MagneticModel.hpp.

Referenced by main().

void GeographicLib::MagneticModel::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
	)		throw () `[static]`

Compute various quantities dependent on the magnetic field and its rate of change.

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 the magnetic field (nT).
[in]	Bxt	the rate of change of Bx (nT/yr).
[in]	Byt	the rate of change of By (nT/yr).
[in]	Bzt	the rate of change of Bz (nT/yr).
[out]	H	the horizontal magnetic field (nT).
[out]	F	the total magnetic field (nT).
[out]	D	the declination of the field (degrees east of north).
[out]	I	the inclination of the field (degrees down from horizontal).
[out]	Ht	the rate of change of H (nT/yr).
[out]	Ft	the rate of change of F (nT/yr).
[out]	Dt	the rate of change of D (degrees/yr).
[out]	It	the rate of change of I (degrees/yr).

Definition at line 222 of file MagneticModel.cpp.

References GeographicLib::Math::hypot(), and GeographicLib::Math::sq().

const std::string& GeographicLib::MagneticModel::Description ( ) const throw () [inline]

Returns:: the description of the magnetic model, if available, from the Description file in the data file; if absent, return "NONE".

Definition at line 231 of file MagneticModel.hpp.

Referenced by main().

const std::string& GeographicLib::MagneticModel::DateTime ( ) const throw () [inline]

Returns:: date of the model, if available, from the ReleaseDate field in the data file; if absent, return "UNKNOWN".

Definition at line 237 of file MagneticModel.hpp.

Referenced by main().

const std::string& GeographicLib::MagneticModel::MagneticFile ( ) const throw () [inline]

Returns:: full file name used to load the magnetic model.

Definition at line 242 of file MagneticModel.hpp.

Referenced by main().

const std::string& GeographicLib::MagneticModel::MagneticModelName ( ) const throw () [inline]

Returns:: "name" used to load the magnetic model (from the first argument of the constructor, but this may be overridden by the model file).

Definition at line 248 of file MagneticModel.hpp.

Referenced by main().

const std::string& GeographicLib::MagneticModel::MagneticModelDirectory ( ) const throw () [inline]

Returns:: directory used to load the magnetic model.

Definition at line 253 of file MagneticModel.hpp.

Math::real GeographicLib::MagneticModel::MinHeight ( ) const throw () [inline]

Returns:: the minimum height above the ellipsoid (in meters) for which this MagneticModel should be used.

Because the model will typically provide useful results slightly outside the range of allowed heights, no check of t argument is made by MagneticModel::operator()() or MagneticModel::Circle.

Definition at line 264 of file MagneticModel.hpp.

Referenced by main().

Math::real GeographicLib::MagneticModel::MaxHeight ( ) const throw () [inline]

Returns:: the maximum height above the ellipsoid (in meters) for which this MagneticModel should be used.

Because the model will typically provide useful results slightly outside the range of allowed heights, no check of t argument is made by MagneticModel::operator()() or MagneticModel::Circle.

Definition at line 275 of file MagneticModel.hpp.

Referenced by main().

Math::real GeographicLib::MagneticModel::MinTime ( ) const throw () [inline]

Returns:: the minimum time (in years) for which this MagneticModel should be used.

Because the model will typically provide useful results slightly outside the range of allowed times, no check of t argument is made by MagneticModel::operator()() or MagneticModel::Circle.

Definition at line 286 of file MagneticModel.hpp.

Referenced by main().

Math::real GeographicLib::MagneticModel::MaxTime ( ) const throw () [inline]

Returns:: the maximum time (in years) for which this MagneticModel should be used.

Because the model will typically provide useful results slightly outside the range of allowed times, no check of t argument is made by MagneticModel::operator()() or MagneticModel::Circle.

Definition at line 297 of file MagneticModel.hpp.

Referenced by main().

Math::real GeographicLib::MagneticModel::MajorRadius ( ) const throw () [inline]

Returns:: a the equatorial radius of the ellipsoid (meters). This is the value of a inherited from the Geocentric object used in the constructor.

Definition at line 304 of file MagneticModel.hpp.

Math::real GeographicLib::MagneticModel::Flattening ( ) const throw () [inline]

Returns:: f the flattening of the ellipsoid. This is the value inherited from the Geocentric object used in the constructor.

Definition at line 310 of file MagneticModel.hpp.

std::string GeographicLib::MagneticModel::DefaultMagneticPath ( ) [static]

Returns:: the default path for magnetic model data files.

This is the value of the environment variable MAGNETIC_PATH, if set; otherwise, it is $GEOGRAPHICLIB_DATA/magnetic if the environment variable GEOGRAPHICLIB_DATA is set; otherwise, it is a compile-time default (/usr/local/share/GeographicLib/magnetic on non-Windows systems and C:/Documents and Settings/All Users/Application Data/GeographicLib/magnetic on Windows systems).

Definition at line 237 of file MagneticModel.cpp.

References GEOGRAPHICLIB_DATA.

Referenced by main(), and MagneticModel().

std::string GeographicLib::MagneticModel::DefaultMagneticName ( ) [static]

Returns:: the default name for the magnetic model.

This is the value of the environment variable MAGNETIC_NAME, if set, otherwise, it is "wmm2010". The MagneticModel class does not use this function; it is just provided as a convenience for a calling program when constructing a MagneticModel object.

Definition at line 250 of file MagneticModel.cpp.

References MAGNETIC_DEFAULT_NAME.

Referenced by main().

The documentation for this class was generated from the following files:

Public Member Functions

Static Public Member Functions

Compute the magnetic field

Detailed Description

Constructor & Destructor Documentation

Member Function Documentation