GeographicLib  1.21
Public Types | Static Public Member Functions | Friends
GeographicLib::GravityModel Class Reference

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

#include <GeographicLib/GravityModel.hpp>

List of all members.

Public Types

enum  mask {
  NONE, GRAVITY, DISTURBANCE, DISTURBING_POTENTIAL,
  SPHERICAL_ANOMALY, GEOID_HEIGHT, ALL
}

Public Member Functions

Setting up the gravity model
 GravityModel (const std::string &name, const std::string &path="")
Compute gravity in geodetic coordinates
Math::real Gravity (real lat, real lon, real h, real &gx, real &gy, real &gz) const throw ()
Math::real Disturbance (real lat, real lon, real h, real &deltax, real &deltay, real &deltaz) const throw ()
Math::real GeoidHeight (real lat, real lon) const throw ()
void SphericalAnomaly (real lat, real lon, real h, real &Dg01, real &xi, real &eta) const throw ()
Compute gravity in geocentric coordinates
Math::real W (real X, real Y, real Z, real &gX, real &gY, real &gZ) const throw ()
Math::real V (real X, real Y, real Z, real &GX, real &GY, real &GZ) const throw ()
Math::real T (real X, real Y, real Z, real &deltaX, real &deltaY, real &deltaZ) const throw ()
Math::real T (real X, real Y, real Z) const throw ()
Math::real U (real X, real Y, real Z, real &gammaX, real &gammaY, real &gammaZ) const throw ()
Math::real Phi (real X, real Y, real &fX, real &fY) const throw ()
Compute gravity on a circle of constant latitude
GravityCircle Circle (real lat, real h, unsigned caps=ALL) const
Inspector functions
const NormalGravityReferenceEllipsoid () const throw ()
const std::string & Description () const throw ()
const std::string & DateTime () const throw ()
const std::string & GravityFile () const throw ()
const std::string & GravityModelName () const throw ()
const std::string & GravityModelDirectory () const throw ()
Math::real MajorRadius () const throw ()
Math::real MassConstant () const throw ()
Math::real ReferenceMassConstant () const throw ()
Math::real AngularVelocity () const throw ()
Math::real Flattening () const throw ()

Static Public Member Functions

static std::string DefaultGravityPath ()
static std::string DefaultGravityName ()

Friends

class GravityCircle

Detailed Description

Model of the earth's gravity field.

Evaluate the earth's gravity field according to a model. The supported models treat only the gravitational field exterior to the mass of the earth. When computing the field at points near (but above) the surface of the earth a small correction can be applied to account for the mass of the atomsphere above the point in question; see The effect of the mass of the atmosphere. Determining the geoid height entails correcting for the mass of the earth above the geoid. The egm96 and egm2008 include separate correction terms to account for this mass.

Definitions and terminology (from Heiskanen and Moritz, Sec 2-13):

See Gravity models for details of how to install the gravity model and the data format.

References:

Example of use:

// Example of using the GeographicLib::GravityModel class
// $Id: fde4f431ed9c43e9e0ec797e302f32a373cdd219 $

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

using namespace std;
using namespace GeographicLib;

int main() {
  try {
    GravityModel grav("egm96");
    double lat = 27.99, lon = 86.93, h = 8820; // Mt Everest
    double gx, gy, gz;
    grav.Gravity(lat,lon, h, gx, gy, gz);
    cout << gx << " " << gy << " " << gz << "\n";
  }
  catch (const exception& e) {
    cerr << "Caught exception: " << e.what() << "\n";
    return 1;
  }
  return 0;
}

Gravity is a command-line utility providing access to the functionality of GravityModel and GravityCircle.


Member Enumeration Documentation

Bit masks for the capabilities to be given to the GravityCircle object produced by Circle.

Enumerator:
NONE 

No capabilities.

GRAVITY 

Allow calls to GravityCircle::Gravity, GravityCircle::W, and GravityCircle::V.

DISTURBANCE 

Allow calls to GravityCircle::Disturbance and GravityCircle::T.

DISTURBING_POTENTIAL 

Allow calls to GravityCircle::T(real lon) (i.e., computing the disturbing potential and not the gravity disturbance vector).

SPHERICAL_ANOMALY 

Allow calls to GravityCircle::SphericalAnomaly.

GEOID_HEIGHT 

Allow calls to GravityCircle::GeoidHeight.

ALL 

All capabilities.

Definition at line 122 of file GravityModel.hpp.


Constructor & Destructor Documentation

GeographicLib::GravityModel::GravityModel ( const std::string &  name,
const std::string &  path = "" 
) [explicit]

Construct a gravity model.

Parameters:
[in]namethe name of the model.
[in]path(optional) directory for data file.

A filename is formed by appending ".egm" (World Gravity 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 DefaultGravityPath(). 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 ".egm.cof").

Definition at line 43 of file GravityModel.cpp.

References DefaultGravityPath(), GeographicLib::SphericalEngine::coeff::readcoeffs(), GeographicLib::SphericalHarmonic::Coefficients(), GeographicLib::SphericalEngine::coeff::nmx(), GeographicLib::Math::sq(), GeographicLib::NormalGravity::MassConstant(), GeographicLib::SphericalEngine::coeff::N(), and GeographicLib::SphericalEngine::coeff::mmx().


Member Function Documentation

Math::real GeographicLib::GravityModel::Gravity ( real  lat,
real  lon,
real  h,
real &  gx,
real &  gy,
real &  gz 
) const throw ()

Evaluate the gravity at an arbitrary point above (or below) the ellipsoid.

Parameters:
[in]latthe geographic latitude (degrees).
[in]lonthe geographic longitude (degrees).
[in]hthe height above the ellipsoid (meters).
[out]gxthe easterly component of the acceleration (m s-2).
[out]gythe northerly component of the acceleration (m s-2).
[out]gzthe upward component of the acceleration (m s-2); this is usually negative.
Returns:
W the sum of the gravitational and centrifugal potentials.

The function includes the effects of the earth's rotation.

Definition at line 291 of file GravityModel.cpp.

Referenced by main().

Math::real GeographicLib::GravityModel::Disturbance ( real  lat,
real  lon,
real  h,
real &  deltax,
real &  deltay,
real &  deltaz 
) const throw ()

Evaluate the gravity disturbance vector at an arbitrary point above (or below) the ellipsoid.

Parameters:
[in]latthe geographic latitude (degrees).
[in]lonthe geographic longitude (degrees).
[in]hthe height above the ellipsoid (meters).
[out]deltaxthe easterly component of the disturbance vector (m s-2).
[out]deltaythe northerly component of the disturbance vector (m s-2).
[out]deltazthe upward component of the disturbance vector (m s-2).
Returns:
T the corresponding disturbing potential.

Definition at line 299 of file GravityModel.cpp.

Referenced by main().

Math::real GeographicLib::GravityModel::GeoidHeight ( real  lat,
real  lon 
) const throw ()

Evaluate the geoid height.

Parameters:
[in]latthe geographic latitude (degrees).
[in]lonthe geographic longitude (degrees).
Returns:
N the height of the geoid above the ReferenceEllipsoid() (meters).

This calls NormalGravity::U for ReferenceEllipsoid(). Some approximations are made in computing the geoid height so that the results of the NGA codes are reproduced accurately. Details are given in Details of the geoid height and anomaly calculations.

Definition at line 277 of file GravityModel.cpp.

References GeographicLib::Math::hypot().

Referenced by main().

void GeographicLib::GravityModel::SphericalAnomaly ( real  lat,
real  lon,
real  h,
real &  Dg01,
real &  xi,
real &  eta 
) const throw ()

Evaluate the components of the gravity anomaly vector using the spherical approximation.

Parameters:
[in]latthe geographic latitude (degrees).
[in]lonthe geographic longitude (degrees).
[in]hthe height above the ellipsoid (meters).
[out]Dg01the gravity anomaly (m s-2).
[out]xithe northerly component of the deflection of the vertical (degrees).
[out]etathe easterly component of the deflection of the vertical (degrees).

The spherical approximation (see Heiskanen and Moritz, Sec 2-14) is used so that the results of the NGA codes are reproduced accurately. approximations used here. Details are given in Details of the geoid height and anomaly calculations.

Definition at line 250 of file GravityModel.cpp.

References GeographicLib::Math::hypot().

Referenced by main().

Math::real GeographicLib::GravityModel::W ( real  X,
real  Y,
real  Z,
real &  gX,
real &  gY,
real &  gZ 
) const throw ()

Evaluate the components of the acceleration due to gravity and the centrifugal acceleration in geocentric coordinates.

Parameters:
[in]Xgeocentric coordinate of point (meters).
[in]Ygeocentric coordinate of point (meters).
[in]Zgeocentric coordinate of point (meters).
[out]gXthe X component of the acceleration (m s-2).
[out]gYthe Y component of the acceleration (m s-2).
[out]gZthe Z component of the acceleration (m s-2).
Returns:
W = V + Phi the sum of the gravitational and centrifugal potentials (m2 s-2).

This calls NormalGravity::U for ReferenceEllipsoid().

Definition at line 241 of file GravityModel.cpp.

Math::real GeographicLib::GravityModel::V ( real  X,
real  Y,
real  Z,
real &  GX,
real &  GY,
real &  GZ 
) const throw ()

Evaluate the components of the acceleration due to gravity in geocentric coordinates.

Parameters:
[in]Xgeocentric coordinate of point (meters).
[in]Ygeocentric coordinate of point (meters).
[in]Zgeocentric coordinate of point (meters).
[out]GXthe X component of the acceleration (m s-2).
[out]GYthe Y component of the acceleration (m s-2).
[out]GZthe Z component of the acceleration (m s-2).
Returns:
V = W - Phi the gravitational potential (m2 s-2).

Definition at line 229 of file GravityModel.cpp.

Math::real GeographicLib::GravityModel::T ( real  X,
real  Y,
real  Z,
real &  deltaX,
real &  deltaY,
real &  deltaZ 
) const throw () [inline]

Evaluate the components of the gravity disturbance in geocentric coordinates.

Parameters:
[in]Xgeocentric coordinate of point (meters).
[in]Ygeocentric coordinate of point (meters).
[in]Zgeocentric coordinate of point (meters).
[out]deltaXthe X component of the gravity disturbance (m s-2).
[out]deltaYthe Y component of the gravity disturbance (m s-2).
[out]deltaZthe Z component of the gravity disturbance (m s-2).
Returns:
T = W - U the disturbing potential (also called the anomalous potential) (m2 s-2).

Definition at line 322 of file GravityModel.hpp.

Math::real GeographicLib::GravityModel::T ( real  X,
real  Y,
real  Z 
) const throw () [inline]

Evaluate disturbing potential in geocentric coordinates.

Parameters:
[in]Xgeocentric coordinate of point (meters).
[in]Ygeocentric coordinate of point (meters).
[in]Zgeocentric coordinate of point (meters).
Returns:
T = W - U the disturbing potential (also called the anomalous potential) (m2 s-2).

Definition at line 335 of file GravityModel.hpp.

Math::real GeographicLib::GravityModel::U ( real  X,
real  Y,
real  Z,
real &  gammaX,
real &  gammaY,
real &  gammaZ 
) const throw () [inline]

Evaluate the components of the acceleration due to normal gravity and the centrifugal acceleration in geocentric coordinates.

Parameters:
[in]Xgeocentric coordinate of point (meters).
[in]Ygeocentric coordinate of point (meters).
[in]Zgeocentric coordinate of point (meters).
[out]gammaXthe X component of the normal acceleration (m s-2).
[out]gammaYthe Y component of the normal acceleration (m s-2).
[out]gammaZthe Z component of the normal acceleration (m s-2).
Returns:
U = V0 + Phi the sum of the normal gravitational and centrifugal potentials (m2 s-2).

This calls NormalGravity::U for ReferenceEllipsoid().

Definition at line 359 of file GravityModel.hpp.

Math::real GeographicLib::GravityModel::Phi ( real  X,
real  Y,
real &  fX,
real &  fY 
) const throw () [inline]

Evaluate the centrifugal acceleration in geocentric coordinates.

Parameters:
[in]Xgeocentric coordinate of point (meters).
[in]Ygeocentric coordinate of point (meters).
[out]fXthe X component of the centrifugal acceleration (m s-2).
[out]fYthe Y component of the centrifugal acceleration (m s-2).
Returns:
Phi the centrifugal potential (m2 s-2).

This calls NormalGravity::Phi for ReferenceEllipsoid().

Definition at line 376 of file GravityModel.hpp.

GravityCircle GeographicLib::GravityModel::Circle ( real  lat,
real  h,
unsigned  caps = ALL 
) const

Create a GravityCircle object to allow the gravity field at many points with constant lat and h and varying lon to be computed efficiently.

Parameters:
[in]latlatitude of the point (degrees).
[in]hthe height of the point above the ellipsoid (meters).
[in]capsbitor'ed combination of GravityModel::mask values specifying the capabilities of the resulting GravityCircle object.
Returns:
a GravityCircle object whose member functions computes the gravitational field at a particular values of lon.

The GravityModel::mask values are

The default value of caps is GravityModel::ALL which turns on all the capabilities. If an unsupported function is invoked, it will return NaNs. Note that GravityModel::GEOID_HEIGHT will only be honored if h = 0.

If the field at several points on a circle of latitude need to be calculated then creating a GravityCircle object and using its member functions will be substantially faster, especially for high-degree models. See Geoid heights on a multi-processor system for an example of using GravityCircle (together with OpenMP) to speed up the computation of geoid heights.

Definition at line 309 of file GravityModel.cpp.

References GeographicLib::NormalGravity::Earth(), GeographicLib::Math::hypot(), GeographicLib::NormalGravity::SurfaceGravity(), GeographicLib::NormalGravity::U(), GeographicLib::NormalGravity::Phi(), GravityCircle, GeographicLib::SphericalHarmonic::Circle(), and GeographicLib::SphericalHarmonic1::Circle().

Referenced by main().

const NormalGravity& GeographicLib::GravityModel::ReferenceEllipsoid ( ) const throw () [inline]
Returns:
the NormalGravity object for the reference ellipsoid.

Definition at line 423 of file GravityModel.hpp.

const std::string& GeographicLib::GravityModel::Description ( ) const throw () [inline]
Returns:
the description of the gravity model, if available, in the data file; if absent, return "NONE".

Definition at line 429 of file GravityModel.hpp.

Referenced by main().

const std::string& GeographicLib::GravityModel::DateTime ( ) const throw () [inline]
Returns:
date of the model; if absent, return "UNKNOWN".

Definition at line 434 of file GravityModel.hpp.

Referenced by main().

const std::string& GeographicLib::GravityModel::GravityFile ( ) const throw () [inline]
Returns:
full file name used to load the gravity model.

Definition at line 439 of file GravityModel.hpp.

Referenced by main().

const std::string& GeographicLib::GravityModel::GravityModelName ( ) const throw () [inline]
Returns:
"name" used to load the gravity model (from the first argument of the constructor, but this may be overridden by the model file).

Definition at line 445 of file GravityModel.hpp.

Referenced by main().

const std::string& GeographicLib::GravityModel::GravityModelDirectory ( ) const throw () [inline]
Returns:
directory used to load the gravity model.

Definition at line 450 of file GravityModel.hpp.

Math::real GeographicLib::GravityModel::MajorRadius ( ) const throw () [inline]
Returns:
a the equatorial radius of the ellipsoid (meters).

Definition at line 455 of file GravityModel.hpp.

Math::real GeographicLib::GravityModel::MassConstant ( ) const throw () [inline]
Returns:
GM the mass constant of the model (m3 s-2); this is the product of G the gravitational constant and M the mass of the earth (usually including the mass of the earth's atmosphere).

Definition at line 463 of file GravityModel.hpp.

Math::real GeographicLib::GravityModel::ReferenceMassConstant ( ) const throw () [inline]
Returns:
GM the mass constant of the ReferenceEllipsoid() (m3 s-2).

Definition at line 469 of file GravityModel.hpp.

Math::real GeographicLib::GravityModel::AngularVelocity ( ) const throw () [inline]
Returns:
omega the angular velocity of the model and the ReferenceEllipsoid() (rad s-1).

Definition at line 476 of file GravityModel.hpp.

Math::real GeographicLib::GravityModel::Flattening ( ) const throw () [inline]
Returns:
f the flattening of the ellipsoid.

Definition at line 482 of file GravityModel.hpp.

std::string GeographicLib::GravityModel::DefaultGravityPath ( ) [static]
Returns:
the default path for gravity model data files.

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

Definition at line 343 of file GravityModel.cpp.

References GEOGRAPHICLIB_DATA.

Referenced by main(), and GravityModel().

std::string GeographicLib::GravityModel::DefaultGravityName ( ) [static]
Returns:
the default name for the gravity model.

This is the value of the environment variable GRAVITY_NAME, if set, otherwise, it is "egm96". The GravityModel class does not use this function; it is just provided as a convenience for a calling program when constructing a GravityModel object.

Definition at line 356 of file GravityModel.cpp.

References GRAVITY_DEFAULT_NAME.

Referenced by main().


Friends And Related Function Documentation

friend class GravityCircle [friend]

Definition at line 90 of file GravityModel.hpp.

Referenced by Circle().


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