GeographicLib  1.35
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GeographicLib::NormalGravity Class Reference

The normal gravity of the earth. More...

#include <GeographicLib/NormalGravity.hpp>

Public Member Functions

Setting up the normal gravity
 NormalGravity (real a, real GM, real omega, real f, real J2)
 
 NormalGravity ()
 
Compute the gravity
Math::real SurfaceGravity (real lat) const throw ()
 
Math::real Gravity (real lat, real h, real &gammay, real &gammaz) const throw ()
 
Math::real U (real X, real Y, real Z, real &gammaX, real &gammaY, real &gammaZ) const throw ()
 
Math::real V0 (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 ()
 
Inspector functions
bool Init () const throw ()
 
Math::real MajorRadius () const throw ()
 
Math::real MassConstant () const throw ()
 
Math::real DynamicalFormFactor (int n=2) const throw ()
 
Math::real AngularVelocity () const throw ()
 
Math::real Flattening () const throw ()
 
Math::real EquatorialGravity () const throw ()
 
Math::real PolarGravity () const throw ()
 
Math::real GravityFlattening () const throw ()
 
Math::real SurfacePotential () const throw ()
 
const GeocentricEarth () const throw ()
 

Static Public Attributes

static const NormalGravity WGS84
 
static const NormalGravity GRS80
 

Friends

class GravityModel
 

Detailed Description

The normal gravity of the earth.

"Normal" gravity refers to an idealization of the earth which is modeled as an rotating ellipsoid. The eccentricity of the ellipsoid, the rotation speed, and the distribution of mass within the ellipsoid are such that the surface of the ellipsoid is a surface of constant potential (gravitational plus centrifugal). The acceleration due to gravity is therefore perpendicular to the surface of the ellipsoid.

There is a closed solution to this problem which is implemented here. Series "approximations" are only used to evaluate certain combinations of elementary functions where use of the closed expression results in a loss of accuracy for small arguments due to cancellation of the two leading terms. However these series include sufficient terms to give full machine precision.

Definitions:

References:

Example of use:

// Example of using the GeographicLib::NormalGravity class
#include <iostream>
#include <exception>
using namespace std;
using namespace GeographicLib;
int main() {
try {
NormalGravity grav(Constants::WGS84_a(), Constants::WGS84_GM<double>(),
Constants::WGS84_omega<double>(),
// Alternatively: const NormalGravity& grav = NormalGravity::WGS84;
double lat = 27.99, h = 8820; // Mt Everest
double gammay, gammaz;
grav.Gravity(lat, h, gammay, gammaz);
cout << gammay << " " << gammaz << "\n";
}
catch (const exception& e) {
cerr << "Caught exception: " << e.what() << "\n";
return 1;
}
return 0;
}

Definition at line 60 of file NormalGravity.hpp.

Constructor & Destructor Documentation

GeographicLib::NormalGravity::NormalGravity ( real  a,
real  GM,
real  omega,
real  f,
real  J2 
)

Constructor for the normal gravity.

Parameters
[in]aequatorial radius (meters).
[in]GMmass constant of the ellipsoid (meters3/seconds2); 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).
[in]omegathe angular velocity (rad s−1).
[in]fthe flattening of the ellipsoid.
[in]J2dynamical form factor.
Exceptions
ifa is not positive or the other constants are inconsistent (see below).

Exactly one of f and J2 should be positive and this will be used to define the ellipsoid. The shape of the ellipsoid can be given in one of two ways:

  • geometrically, the ellipsoid is defined by the flattening f = (ab) / a, where a and b are the equatorial radius and the polar semi-axis.
  • physically, the ellipsoid is defined by the dynamical form factor J2 = (CA) / Ma2, where A and C are the equatorial and polar moments of inertia and M is the mass of the earth.

Definition at line 16 of file NormalGravity.cpp.

References GeographicLib::Math::isfinite().

GeographicLib::NormalGravity::NormalGravity ( )
inline

A default constructor for the normal gravity. This sets up an uninitialized object and is used by GravityModel which constructs this object before it has read in the parameters for the reference ellipsoid.

Definition at line 108 of file NormalGravity.hpp.

Member Function Documentation

Math::real GeographicLib::NormalGravity::SurfaceGravity ( real  lat) const
throw (
)

Evaluate the gravity on the surface of the ellipsoid.

Parameters
[in]latthe geographic latitude (degrees).
Returns
γ the acceleration due to gravity, positive downwards (m s−2).

Due to the axial symmetry of the ellipsoid, the result is independent of the value of the longitude. This acceleration is perpendicular to the surface of the ellipsoid. It includes the effects of the earth's rotation.

Definition at line 149 of file NormalGravity.cpp.

References GeographicLib::Math::sq().

Referenced by GeographicLib::GravityModel::Circle().

Math::real GeographicLib::NormalGravity::Gravity ( real  lat,
real  h,
real &  gammay,
real &  gammaz 
) const
throw (
)

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

Parameters
[in]latthe geographic latitude (degrees).
[in]hthe height above the ellipsoid (meters).
[out]gammaythe northerly component of the acceleration (m s−2).
[out]gammazthe upward component of the acceleration (m s−2); this is usually negative.
Returns
U the corresponding normal potential.

Due to the axial symmetry of the ellipsoid, the result is independent of the value of the longitude and the easterly component of the acceleration vanishes, gammax = 0. The function includes the effects of the earth's rotation. When h = 0, this function gives gammay = 0 and the returned value matches that of NormalGravity::SurfaceGravity.

Definition at line 220 of file NormalGravity.cpp.

Math::real GeographicLib::NormalGravity::U ( real  X,
real  Y,
real  Z,
real &  gammaX,
real &  gammaY,
real &  gammaZ 
) 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]gammaXthe X component of the acceleration (m s−2).
[out]gammaYthe Y component of the acceleration (m s−2).
[out]gammaZthe Z component of the acceleration (m s−2).
Returns
U = V0 + Φ the sum of the gravitational and centrifugal potentials (m2 s−2).

The acceleration given by γ = ∇U = ∇V0 + ∇Φ = Γ + f.

Definition at line 210 of file NormalGravity.cpp.

Referenced by GeographicLib::GravityModel::Circle().

Math::real GeographicLib::NormalGravity::V0 ( real  X,
real  Y,
real  Z,
real &  GammaX,
real &  GammaY,
real &  GammaZ 
) const
throw (
)

Evaluate the components of the acceleration due to gravity alone 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 acceleration due to gravity (m s−2).
[out]GammaYthe Y component of the acceleration due to gravity (m s−2).
[out]GammaZthe Z component of the acceleration due to gravity (m s−2).
Returns
V0 the gravitational potential (m2 s−2).

This function excludes the centrifugal acceleration and is appropriate to use for space applications. In terrestrial applications, the function NormalGravity::U (which includes this effect) should usually be used.

Definition at line 157 of file NormalGravity.cpp.

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

Math::real GeographicLib::NormalGravity::Phi ( real  X,
real  Y,
real &  fX,
real &  fY 
) const
throw (
)

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
Φ the centrifugal potential (m2 s−2).

Φ is independent of Z, thus fZ = 0. This function NormalGravity::U sums the results of NormalGravity::V0 and NormalGravity::Phi.

Definition at line 202 of file NormalGravity.cpp.

References GeographicLib::Math::sq().

Referenced by GeographicLib::GravityModel::Circle().

bool GeographicLib::NormalGravity::Init ( ) const
throw (
)
inline
Returns
true if the object has been initialized.

Definition at line 221 of file NormalGravity.hpp.

Math::real GeographicLib::NormalGravity::MajorRadius ( ) const
throw (
)
inline
Returns
a the equatorial radius of the ellipsoid (meters). This is the value used in the constructor.

Definition at line 227 of file NormalGravity.hpp.

Math::real GeographicLib::NormalGravity::MassConstant ( ) const
throw (
)
inline
Returns
GM the mass constant of the ellipsoid (m3 s−2). This is the value used in the constructor.

Definition at line 235 of file NormalGravity.hpp.

Referenced by GeographicLib::GravityModel::GravityModel().

Math::real GeographicLib::NormalGravity::DynamicalFormFactor ( int  n = 2) const
throw (
)
inline
Returns
Jn the dynamical form factors of the ellipsoid.

If n = 2 (the default), this is the value of J2 used in the constructor. Otherwise it is the zonal coefficient of the Legendre harmonic sum of the normal gravitational potential. Note that Jn = 0 if n is odd. In most gravity applications, fully normalized Legendre functions are used and the corresponding coefficient is Cn0 = −Jn / sqrt(2 n + 1).

Definition at line 249 of file NormalGravity.hpp.

Math::real GeographicLib::NormalGravity::AngularVelocity ( ) const
throw (
)
inline
Returns
ω the angular velocity of the ellipsoid (rad s−1). This is the value used in the constructor.

Definition at line 256 of file NormalGravity.hpp.

Math::real GeographicLib::NormalGravity::Flattening ( ) const
throw (
)
inline
Returns
f the flattening of the ellipsoid (ab)/a.

Definition at line 263 of file NormalGravity.hpp.

Math::real GeographicLib::NormalGravity::EquatorialGravity ( ) const
throw (
)
inline
Returns
γe the normal gravity at equator (m s−2).

Definition at line 270 of file NormalGravity.hpp.

Math::real GeographicLib::NormalGravity::PolarGravity ( ) const
throw (
)
inline
Returns
γp the normal gravity at poles (m s−2).

Definition at line 277 of file NormalGravity.hpp.

Math::real GeographicLib::NormalGravity::GravityFlattening ( ) const
throw (
)
inline
Returns
f* the gravity flattening (γp − γe) / γe.

Definition at line 284 of file NormalGravity.hpp.

Math::real GeographicLib::NormalGravity::SurfacePotential ( ) const
throw (
)
inline
Returns
U0 the constant normal potential for the surface of the ellipsoid (m2 s−2).

Definition at line 291 of file NormalGravity.hpp.

const Geocentric& GeographicLib::NormalGravity::Earth ( ) const
throw (
)
inline
Returns
the Geocentric object used by this instance.

Definition at line 297 of file NormalGravity.hpp.

Referenced by GeographicLib::GravityModel::Circle().

Friends And Related Function Documentation

friend class GravityModel
friend

Definition at line 64 of file NormalGravity.hpp.

Member Data Documentation

const NormalGravity GeographicLib::NormalGravity::WGS84
static

A global instantiation of NormalGravity for the WGS84 ellipsoid.

Definition at line 303 of file NormalGravity.hpp.

const NormalGravity GeographicLib::NormalGravity::GRS80
static

A global instantiation of NormalGravity for the GRS80 ellipsoid.

Definition at line 308 of file NormalGravity.hpp.


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