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GeographicLib::AuxLatitude Class Reference

Conversions between auxiliary latitudes. More...

`#include <GeographicLib/AuxLatitude.hpp>`

Inheritance diagram for GeographicLib::AuxLatitude:

## Public Types

enum | aux { GEOGRAPHIC , PARAMETRIC , GEOCENTRIC , RECTIFYING , CONFORMAL , AUTHALIC , AUXNUMBER , PHI , BETA , THETA , MU , CHI , XI , COMMON , GEODETIC , REDUCED }

## Public Member Functions

| AuxLatitude (real a, real f)
AuxAngle | Convert (int auxin, int auxout, const AuxAngle &zeta, bool exact=false) const
Math::real | Convert (int auxin, int auxout, real zeta, bool exact=false) const
AuxAngle | ToAuxiliary (int auxout, const AuxAngle &phi, real *diff=nullptr) const
AuxAngle | FromAuxiliary (int auxin, const AuxAngle &zeta, int *niter=nullptr) const
Math::real | RectifyingRadius (bool exact=false) const
Math::real | AuthalicRadiusSquared (bool exact=false) const
Math::real | EquatorialRadius () const
Math::real | PolarSemiAxis () const
Math::real | Flattening () const

## Static Public Member Functions

static AuxLatitude | axes (real a, real b)
static Math::real | Clenshaw (bool sinp, real szeta, real czeta, const real c[], int K)
static const AuxLatitude & | WGS84 ()

## Static Public Attributes

static const int | Lmax

## Protected Member Functions

AuxAngle | Parametric (const AuxAngle &phi, real *diff=nullptr) const
AuxAngle | Geocentric (const AuxAngle &phi, real *diff=nullptr) const
AuxAngle | Rectifying (const AuxAngle &phi, real *diff=nullptr) const
AuxAngle | Conformal (const AuxAngle &phi, real *diff=nullptr) const
AuxAngle | Authalic (const AuxAngle &phi, real *diff=nullptr) const

## Detailed Description

Conversions between auxiliary latitudes.

This class is an implementation of the methods described in

- C. F. F. Karney, On auxiliary latitudes, Survey Review 56(395), 165–180 (2024); preprint arXiv:2212.05818.

The provides accurate conversions between geographic (phi, φ), parametric (beta, β), geocentric (theta, θ), rectifying (mu, μ), conformal (chi, χ), and authalic (xi, ξ) latitudes for an ellipsoid of revolution. A latitude is represented by the class AuxAngle in order to maintain precision close to the poles.

The class implements two methods for the conversion:

- Direct evaluation of the defining equations, the exact method. These equations are formulated so as to preserve relative accuracy of the tangent of the latitude, ensuring high accuracy near the equator and the poles. Newton's method is used for those conversions that can't be expressed in closed form.
- Expansions in powers of n, the third flattening, the series method. This delivers full accuracy for abs(f) ≤ 1/150. Here, f is the flattening of the ellipsoid.

The series method is the preferred method of conversion for any conversion involving μ, χ, or ξ, with abs(f) ≤ 1/150. The equations for the conversions between φ, β, and θ are sufficiently simple that the exact method should be used for such conversions and also for conversions with with abs(f) > 1/150.

Example of use:

// Example of using the GeographicLib::AuxLatitude class. See the paper
//
// - C. F. F. Karney,
// On auxiliary latitudes,
// Survey Review 56(395), 165--180 (2024).
// https://doi.org/10.1080/00396265.2023.2217604
// preprint: https://arxiv.org/abs/2212.05818
#include <iostream>
#include <iomanip>
#include <exception>
#include <GeographicLib/Utility.hpp>
#include <GeographicLib/AuxLatitude.hpp>
int main(int argc, const char* const argv[]) {
try {
using latitude = GeographicLib::AuxLatitude;
using angle = GeographicLib::AuxAngle;
if (argc != 3) {
std::cerr << "Usage: example-AuxLatitude <n> <base-lat>\n";
return 1;
}
double n = GeographicLib::Utility::fract<double>(std::string(argv[1]));
int auxin = GeographicLib::Utility::val<int>(std::string(argv[2]));
double a = 1+n, b = 1-n; // Equatorial radius and polar semiaxis
latitude aux(latitude::axes(a, b));
bool exact = true; // Do the exact calculation
std::cout << std::setprecision(9) << std::fixed;
int m = 1;
for (int l = 0; l < 90*m; ++l) {
angle phi(angle::degrees((l+0.5)/m));
for (int auxout = 0; auxout < latitude::AUXNUMBER; ++auxout) {
angle eta = aux.Convert(auxin, auxout, phi, exact);
std::cout << (auxout ? " " : "") << eta.degrees();
}
std::cout << "\n";
}
}
catch (const std::exception& e) {
std::cerr << "Caught exception: " << e.what() << "\n";
return 1;
}
}
AuxLatitude.hpp
Header for the GeographicLib::AuxLatitude class.
main
int main(int argc, const char *const argv[])
Definition Cart3Convert.cpp:52
Utility.hpp
Header for GeographicLib::Utility class.
GeographicLib::AuxAngle
An accurate representation of angles.
Definition AuxAngle.hpp:51
GeographicLib::AuxLatitude
Conversions between auxiliary latitudes.
Definition AuxLatitude.hpp:75
GeographicLib::Utility::fract
static T fract(const std::string &s)
Definition Utility.hpp:298
GeographicLib::Utility::val
static T val(const std::string &s)
Definition Utility.hpp:225

Definition at line 75 of file AuxLatitude.hpp.

## Member Enumeration Documentation

## aux

enum GeographicLib::AuxLatitude::aux

The floating-point type for real numbers. This just connects to the template parameters for the class. The different auxiliary latitudes.

Enumerator
GEOGRAPHIC | Geographic latitude, phi, φ
PARAMETRIC | Parametric latitude, beta, β
GEOCENTRIC | Geocentric latitude, theta, θ
RECTIFYING | Rectifying latitude, mu, μ
CONFORMAL | Conformal latitude, chi, χ
AUTHALIC | Authalic latitude, xi, ξ
AUXNUMBER | The total number of auxiliary latitudes
PHI | An alias for GEOGRAPHIC
BETA | An alias for PARAMETRIC
THETA | An alias for GEOCENTRIC
MU | An alias for RECTIFYING
CHI | An alias for CONFORMAL
XI | An alias for AUTHALIC
COMMON | An alias for GEOGRAPHIC
GEODETIC | An alias for GEOGRAPHIC
REDUCED | An alias for PARAMETRIC

Definition at line 86 of file AuxLatitude.hpp.

## Constructor & Destructor Documentation

## AuxLatitude()

GeographicLib::AuxLatitude::AuxLatitude | ( | real | a,
|  | real | f )

Constructor

Parameters

  [in] | a | equatorial radius.
  [in] | f | flattening of ellipsoid. Setting f = 0 gives a sphere. Negative f gives a prolate ellipsoid.

Exceptions

  GeographicErr | if a or (1 − f) a is not positive.

Note
  the constructor does not precompute the coefficients for the Fourier series for the series conversions. These are computed and saved when first needed.

Definition at line 25 of file AuxLatitude.cpp.

References AUXNUMBER, and Lmax.

## Member Function Documentation

## axes()

AuxLatitude GeographicLib::AuxLatitude::axes | ( | real | a,
|  | real | b )

inlinestatic

Construct and return an AuxLatitude object specified in terms of the semiaxes

Parameters

  [in] | a | equatorial radius.
  [in] | b | polar semiaxis.

Exceptions

  GeographicErr | if a or b is not positive.

This allows a new AuxAngle to be initialized as an angle in radians with

AuxLatitude aux(AuxLatitude::axes(a, b));
GeographicLib::AuxLatitude::aux
aux
Definition AuxLatitude.hpp:86
GeographicLib::AuxLatitude::axes
static AuxLatitude axes(real a, real b)
Definition AuxLatitude.hpp:195

Definition at line 195 of file AuxLatitude.hpp.

## Convert() [1/2]

AuxAngle GeographicLib::AuxLatitude::Convert | ( | int | auxin,
|  | int | auxout,
|  | const AuxAngle & | zeta,
|  | bool | exact = false ) const

Convert between any two auxiliary latitudes specified as AuxAngle.

Parameters

  [in] | auxin | an AuxLatitude::aux indicating the type of auxiliary latitude zeta.
  [in] | auxout | an AuxLatitude::aux indicating the type of auxiliary latitude eta.
  [in] | zeta | the input auxiliary latitude as an AuxAngle
  [in] | exact | if true use the exact equations instead of the Taylor series [default false].

Returns
  the output auxiliary latitude eta as an AuxAngle.

With exact = false, the Fourier coefficients for a specific auxin and auxout are computed and saved on the first call; the saved coefficients are used on subsequent calls. The series method is accurate for abs(f) ≤ 1/150; for other f, the exact method should be used.

Definition at line 294 of file AuxLatitude.cpp.

References Clenshaw(), FromAuxiliary(), Lmax, GeographicLib::AuxAngle::NaN(), GeographicLib::AuxAngle::normalized(), GeographicLib::AuxAngle::radians(), ToAuxiliary(), GeographicLib::AuxAngle::x(), and GeographicLib::AuxAngle::y().

Referenced by Convert(), and main().

## Convert() [2/2]

Math::real GeographicLib::AuxLatitude::Convert | ( | int | auxin,
|  | int | auxout,
|  | real | zeta,
|  | bool | exact = false ) const

Convert between any two auxiliary latitudes specified in degrees.

Parameters

  [in] | auxin | an AuxLatitude::aux indicating the type of auxiliary latitude zeta.
  [in] | auxout | an AuxLatitude::aux indicating the type of auxiliary latitude eta.
  [in] | zeta | the input auxiliary latitude in degrees.
  [in] | exact | if true use the exact equations instead of the Taylor series [default false].

Returns
  the output auxiliary latitude eta in degrees.

With exact = false, the Fourier coefficients for a specific auxin and auxout are computed and saved on the first call; the saved coefficients are used on subsequent calls. The series method is accurate for abs(f) ≤ 1/150; for other f, the exact method should be used.

Definition at line 314 of file AuxLatitude.cpp.

References Convert(), GeographicLib::AuxAngle::degrees(), and GeographicLib::Math::td.

## ToAuxiliary()

AuxAngle GeographicLib::AuxLatitude::ToAuxiliary | ( | int | auxout,
|  | const AuxAngle & | phi,
|  | real * | diff = nullptr ) const

Convert geographic latitude to an auxiliary latitude eta.

Parameters

  [in] | auxout | an AuxLatitude::aux indicating the auxiliary latitude returned.
  [in] | phi | the geographic latitude.
  [out] | diff | optional pointer to the derivative d tan(eta) / d tan(phi).

Returns
  the auxiliary latitude eta.

This uses the exact equations.

Definition at line 217 of file AuxLatitude.cpp.

References AUTHALIC, Authalic(), CONFORMAL, Conformal(), GEOCENTRIC, Geocentric(), GEOGRAPHIC, GeographicLib::AuxAngle::NaN(), PARAMETRIC, Parametric(), RECTIFYING, and Rectifying().

Referenced by Convert(), and FromAuxiliary().

## FromAuxiliary()

AuxAngle GeographicLib::AuxLatitude::FromAuxiliary | ( | int | auxin,
|  | const AuxAngle & | zeta,
|  | int * | niter = nullptr ) const

Convert an auxiliary latitude zeta to geographic latitude.

Parameters

  [in] | auxin | an AuxLatitude::aux indicating the type of auxiliary latitude zeta.
  [in] | zeta | the input auxiliary latitude.
  [out] | niter | optional pointer to the number of iterations.

Returns
  the geographic latitude phi.

This uses the exact equations.

Definition at line 233 of file AuxLatitude.cpp.

References AUTHALIC, CONFORMAL, GeographicLib::AuxAngle::copyquadrant(), GEOCENTRIC, GEOGRAPHIC, GeographicLib::AuxAngle::NaN(), PARAMETRIC, RECTIFYING, GeographicLib::AuxAngle::tan(), ToAuxiliary(), GeographicLib::AuxAngle::x(), and GeographicLib::AuxAngle::y().

Referenced by Convert().

## RectifyingRadius()

Math::real GeographicLib::AuxLatitude::RectifyingRadius | ( | bool | exact = false | ) | const

Return the rectifying radius.

Parameters

  [in] | exact | if true use the exact expression instead of the Taylor series [default false].

Returns
  the rectifying radius in the same units as a.

Definition at line 321 of file AuxLatitude.cpp.

References Lmax, GeographicLib::Math::pi(), GeographicLib::Math::polyval(), GeographicLib::EllipticFunction::RG(), and GeographicLib::Math::sq().

Referenced by GeographicLib::DAuxLatitude::DRectifying().

## AuthalicRadiusSquared()

Math::real GeographicLib::AuxLatitude::AuthalicRadiusSquared | ( | bool | exact = false | ) | const

Return the authalic radius squared.

Parameters

  [in] | exact | if true use the exact expression instead of the Taylor series [default false].

Returns
  the authalic radius squared in the same units as a.

Definition at line 348 of file AuxLatitude.cpp.

References Lmax, GeographicLib::Math::polyval(), and GeographicLib::Math::sq().

Referenced by main().

## EquatorialRadius()

Math::real GeographicLib::AuxLatitude::EquatorialRadius | ( |  | ) | const

inline

Returns
  a the equatorial radius of the ellipsoid (meters).

Definition at line 284 of file AuxLatitude.hpp.

## PolarSemiAxis()

Math::real GeographicLib::AuxLatitude::PolarSemiAxis | ( |  | ) | const

inline

Returns
  b the polar semiaxis of the ellipsoid (meters).

Definition at line 288 of file AuxLatitude.hpp.

## Flattening()

Math::real GeographicLib::AuxLatitude::Flattening | ( |  | ) | const

inline

Returns
  f, the flattening of the ellipsoid.

Definition at line 292 of file AuxLatitude.hpp.

## Clenshaw()

Math::real GeographicLib::AuxLatitude::Clenshaw | ( | bool | sinp,
|  | real | szeta,
|  | real | czeta,
|  | const real | c[],
|  | int | K )

static

Use Clenshaw to sum a Fouier series.

Parameters

  [in] | sinp | if true sum the sine series, else sum the cosine series.
  [in] | szeta | sin(zeta).
  [in] | czeta | cos(zeta).
  [in] | c | the array of coefficients.
  [in] | K | the number of coefficients.

Returns
  the Clenshaw sum.

The result returned is \( \sum_0^{K-1} c_k \sin (2k+2)\zeta \), if sinp is true; with sinp false, replace sin by cos.

Referenced by Convert().

## WGS84()

const AuxLatitude & GeographicLib::AuxLatitude::WGS84 | ( |  | ) |

static

A global instantiation of Ellipsoid with the parameters for the WGS84 ellipsoid.

Definition at line 79 of file AuxLatitude.cpp.

References GeographicLib::Constants::WGS84_a(), and GeographicLib::Constants::WGS84_f().

## Parametric()

AuxAngle GeographicLib::AuxLatitude::Parametric | ( | const AuxAngle & | phi,
|  | real * | diff = nullptr ) const

protected

Convert geographic latitude to parametric latitude

Parameters

  [in] | phi | geographic latitude.
  [out] | diff | optional pointer to the derivative d tan(beta) / d tan(phi).

Returns
  beta, the parametric latitude

Definition at line 84 of file AuxLatitude.cpp.

References GeographicLib::AuxAngle::x(), and GeographicLib::AuxAngle::y().

Referenced by Authalic(), Conformal(), GeographicLib::DAuxLatitude::DRectifying(), Rectifying(), and ToAuxiliary().

## Geocentric()

AuxAngle GeographicLib::AuxLatitude::Geocentric | ( | const AuxAngle & | phi,
|  | real * | diff = nullptr ) const

protected

Convert geographic latitude to geocentric latitude

Parameters

  [in] | phi | geographic latitude.
  [out] | diff | optional pointer to the derivative d tan(theta) / d tan(phi).

Returns
  theta, the geocentric latitude.

Definition at line 89 of file AuxLatitude.cpp.

References GeographicLib::AuxAngle::x(), and GeographicLib::AuxAngle::y().

Referenced by ToAuxiliary().

## Rectifying()

AuxAngle GeographicLib::AuxLatitude::Rectifying | ( | const AuxAngle & | phi,
|  | real * | diff = nullptr ) const

protected

Convert geographic latitude to rectifying latitude

Parameters

  [in] | phi | geographic latitude.
  [out] | diff | optional pointer to the derivative d tan(mu) / d tan(phi).

Returns
  mu, the rectifying latitude.

Definition at line 94 of file AuxLatitude.cpp.

References GeographicLib::AuxAngle::normalized(), Parametric(), GeographicLib::Math::pi(), GeographicLib::EllipticFunction::RD(), GeographicLib::EllipticFunction::RF(), GeographicLib::Math::sq(), std::swap(), GeographicLib::AuxAngle::tan(), GeographicLib::AuxAngle::x(), and GeographicLib::AuxAngle::y().

Referenced by GeographicLib::DAuxLatitude::DRectifying(), and ToAuxiliary().

## Conformal()

AuxAngle GeographicLib::AuxLatitude::Conformal | ( | const AuxAngle & | phi,
|  | real * | diff = nullptr ) const

protected

Convert geographic latitude to conformal latitude

Parameters

  [in] | phi | geographic latitude.
  [out] | diff | optional pointer to the derivative d tan(chi) / d tan(phi).

Returns
  chi, the conformal latitude.

Definition at line 139 of file AuxLatitude.cpp.

References GeographicLib::AuxAngle::normalized(), Parametric(), GeographicLib::AuxAngle::tan(), and GeographicLib::AuxAngle::x().

Referenced by ToAuxiliary().

## Authalic()

AuxAngle GeographicLib::AuxLatitude::Authalic | ( | const AuxAngle & | phi,
|  | real * | diff = nullptr ) const

protected

Convert geographic latitude to authalic latitude

Parameters

  [in] | phi | geographic latitude.
  [out] | diff | optional pointer to the derivative d tan(xi) / d tan(phi).

Returns
  xi, the authalic latitude.

Definition at line 196 of file AuxLatitude.cpp.

References GeographicLib::AuxAngle::normalized(), Parametric(), GeographicLib::Math::sq(), GeographicLib::AuxAngle::tan(), GeographicLib::AuxAngle::x(), and GeographicLib::AuxAngle::y().

Referenced by ToAuxiliary().

## Member Data Documentation

## Lmax

const int GeographicLib::AuxLatitude::Lmax

static

The order of the series expansions. This is set at compile time to either 4, 6, or 8, by the preprocessor macro GEOGRAPHICLIB_AUXLATITUDE_ORDER.

Definition at line 316 of file AuxLatitude.hpp.

Referenced by AuthalicRadiusSquared(), AuxLatitude(), Convert(), GeographicLib::DAuxLatitude::DConvert(), and RectifyingRadius().

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

- AuxLatitude.hpp
- AuxLatitude.cpp