GeographicLib  1.35
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GeographicLib::Gnomonic Class Reference

Gnomonic projection More...

#include <GeographicLib/Gnomonic.hpp>

Public Member Functions

 Gnomonic (const Geodesic &earth=Geodesic::WGS84) throw ()
 
void Forward (real lat0, real lon0, real lat, real lon, real &x, real &y, real &azi, real &rk) const throw ()
 
void Reverse (real lat0, real lon0, real x, real y, real &lat, real &lon, real &azi, real &rk) const throw ()
 
void Forward (real lat0, real lon0, real lat, real lon, real &x, real &y) const throw ()
 
void Reverse (real lat0, real lon0, real x, real y, real &lat, real &lon) const throw ()
 
Inspector functions
Math::real MajorRadius () const throw ()
 
Math::real Flattening () const throw ()
 

Detailed Description

Gnomonic projection

Gnomonic projection centered at an arbitrary position C on the ellipsoid. This projection is derived in Section 8 of

The projection of P is defined as follows: compute the geodesic line from C to P; compute the reduced length m12, geodesic scale M12, and ρ = m12/M12; finally x = ρ sin azi1; y = ρ cos azi1, where azi1 is the azimuth of the geodesic at C. The Gnomonic::Forward and Gnomonic::Reverse methods also return the azimuth azi of the geodesic at P and reciprocal scale rk in the azimuthal direction. The scale in the radial direction if 1/rk2.

For a sphere, ρ is reduces to a tan(s12/a), where s12 is the length of the geodesic from C to P, and the gnomonic projection has the property that all geodesics appear as straight lines. For an ellipsoid, this property holds only for geodesics interesting the centers. However geodesic segments close to the center are approximately straight.

Consider a geodesic segment of length l. Let T be the point on the geodesic (extended if necessary) closest to C the center of the projection and t be the distance CT. To lowest order, the maximum deviation (as a true distance) of the corresponding gnomonic line segment (i.e., with the same end points) from the geodesic is

(K(T) - K(C)) l2 t / 32.

where K is the Gaussian curvature.

This result applies for any surface. For an ellipsoid of revolution, consider all geodesics whose end points are within a distance r of C. For a given r, the deviation is maximum when the latitude of C is 45°, when endpoints are a distance r away, and when their azimuths from the center are ± 45° or ± 135°. To lowest order in r and the flattening f, the deviation is f (r/2a)3 r.

The conversions all take place using a Geodesic object (by default Geodesic::WGS84). For more information on geodesics see geodesic.

CAUTION: The definition of this projection for a sphere is standard. However, there is no standard for how it should be extended to an ellipsoid. The choices are:

Example of use:

// Example of using the GeographicLib::Gnomonic class
#include <iostream>
#include <exception>
using namespace std;
using namespace GeographicLib;
int main() {
try {
// Alternatively: const Geodesic& geod = Geodesic::WGS84;
const double lat0 = 48 + 50/60.0, lon0 = 2 + 20/60.0; // Paris
Gnomonic proj(geod);
{
// Sample forward calculation
double lat = 50.9, lon = 1.8; // Calais
double x, y;
proj.Forward(lat0, lon0, lat, lon, x, y);
cout << x << " " << y << "\n";
}
{
// Sample reverse calculation
double x = -38e3, y = 230e3;
double lat, lon;
proj.Reverse(lat0, lon0, x, y, lat, lon);
cout << lat << " " << lon << "\n";
}
}
catch (const exception& e) {
cerr << "Caught exception: " << e.what() << "\n";
return 1;
}
return 0;
}

GeodesicProj is a command-line utility providing access to the functionality of AzimuthalEquidistant, Gnomonic, and CassiniSoldner.

Definition at line 101 of file Gnomonic.hpp.

Constructor & Destructor Documentation

GeographicLib::Gnomonic::Gnomonic ( const Geodesic earth = Geodesic::WGS84)
throw (
)
inlineexplicit

Constructor for Gnomonic.

Parameters
[in]earththe Geodesic object to use for geodesic calculations. By default this uses the WGS84 ellipsoid.

Definition at line 117 of file Gnomonic.hpp.

Member Function Documentation

void GeographicLib::Gnomonic::Forward ( real  lat0,
real  lon0,
real  lat,
real  lon,
real &  x,
real &  y,
real &  azi,
real &  rk 
) const
throw (
)

Forward projection, from geographic to gnomonic.

Parameters
[in]lat0latitude of center point of projection (degrees).
[in]lon0longitude of center point of projection (degrees).
[in]latlatitude of point (degrees).
[in]lonlongitude of point (degrees).
[out]xeasting of point (meters).
[out]ynorthing of point (meters).
[out]aziazimuth of geodesic at point (degrees).
[out]rkreciprocal of azimuthal scale at point.

lat0 and lat should be in the range [−90°, 90°] and lon0 and lon should be in the range [−540°, 540°). The scale of the projection is 1/rk2 in the "radial" direction, azi clockwise from true north, and is 1/rk in the direction perpendicular to this. If the point lies "over the horizon", i.e., if rk ≤ 0, then NaNs are returned for x and y (the correct values are returned for azi and rk). A call to Forward followed by a call to Reverse will return the original (lat, lon) (to within roundoff) provided the point in not over the horizon.

Definition at line 24 of file Gnomonic.cpp.

References GeographicLib::Geodesic::AZIMUTH, GeographicLib::Geodesic::GEODESICSCALE, and GeographicLib::Geodesic::REDUCEDLENGTH.

Referenced by main().

void GeographicLib::Gnomonic::Reverse ( real  lat0,
real  lon0,
real  x,
real  y,
real &  lat,
real &  lon,
real &  azi,
real &  rk 
) const
throw (
)

Reverse projection, from gnomonic to geographic.

Parameters
[in]lat0latitude of center point of projection (degrees).
[in]lon0longitude of center point of projection (degrees).
[in]xeasting of point (meters).
[in]ynorthing of point (meters).
[out]latlatitude of point (degrees).
[out]lonlongitude of point (degrees).
[out]aziazimuth of geodesic at point (degrees).
[out]rkreciprocal of azimuthal scale at point.

lat0 should be in the range [−90°, 90°] and lon0 should be in the range [−540°, 540°). lat will be in the range [−90°, 90°] and lon will be in the range [−180°, 180°). The scale of the projection is 1/rk2 in the "radial" direction, azi clockwise from true north, and is 1/rk in the direction perpendicular to this. Even though all inputs should return a valid lat and lon, it's possible that the procedure fails to converge for very large x or y; in this case NaNs are returned for all the output arguments. A call to Reverse followed by a call to Forward will return the original (x, y) (to roundoff).

Definition at line 43 of file Gnomonic.cpp.

References GeographicLib::Geodesic::AZIMUTH, GeographicLib::Geodesic::DISTANCE_IN, GeographicLib::Geodesic::GEODESICSCALE, GeographicLib::Math::hypot(), GeographicLib::Geodesic::LATITUDE, GeographicLib::Geodesic::LONGITUDE, GeographicLib::GeodesicLine::Position(), and GeographicLib::Geodesic::REDUCEDLENGTH.

Referenced by main().

void GeographicLib::Gnomonic::Forward ( real  lat0,
real  lon0,
real  lat,
real  lon,
real &  x,
real &  y 
) const
throw (
)
inline

Gnomonic::Forward without returning the azimuth and scale.

Definition at line 179 of file Gnomonic.hpp.

void GeographicLib::Gnomonic::Reverse ( real  lat0,
real  lon0,
real  x,
real  y,
real &  lat,
real &  lon 
) const
throw (
)
inline

Gnomonic::Reverse without returning the azimuth and scale.

Definition at line 188 of file Gnomonic.hpp.

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

Definition at line 201 of file Gnomonic.hpp.

Math::real GeographicLib::Gnomonic::Flattening ( ) const
throw (
)
inline
Returns
f the flattening of the ellipsoid. This is the value inherited from the Geodesic object used in the constructor.

Definition at line 207 of file Gnomonic.hpp.


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