Looking up the height of the geoid above the ellipsoid. More...

#include <GeographicLib/Geoid.hpp>

Public Types
enum	convertflag { ELLIPSOIDTOGEOID = -1 , NONE = 0 , GEOIDTOELLIPSOID = 1 }

Public Member Functions
Setting up the geoid
	Geoid (const std::string &name, const std::string &path="", bool cubic=true, bool threadsafe=false)

void	CacheArea (real south, real west, real north, real east) const

void	CacheAll () const

void	CacheClear () const

Compute geoid heights
Math::real	operator() (real lat, real lon) const

Math::real	ConvertHeight (real lat, real lon, real h, convertflag d) const

Inspector functions
const std::string &	Description () const

const std::string &	DateTime () const

const std::string &	GeoidFile () const

const std::string &	GeoidName () const

const std::string &	GeoidDirectory () const

const std::string	Interpolation () const

Math::real	MaxError () const

Math::real	RMSError () const

Math::real	Offset () const

Math::real	Scale () const

bool	ThreadSafe () const

bool	Cache () const

Math::real	CacheWest () const

Math::real	CacheEast () const

Math::real	CacheNorth () const

Math::real	CacheSouth () const

Math::real	EquatorialRadius () const

Math::real	Flattening () const

Static Public Member Functions
static std::string	DefaultGeoidPath ()

static std::string	DefaultGeoidName ()

Detailed Description

Looking up the height of the geoid above the ellipsoid.

This class evaluates the height of one of the standard geoids, EGM84, EGM96, or EGM2008 by bilinear or cubic interpolation into a rectangular grid of data. These geoid models are documented in

The geoids are defined in terms of spherical harmonics. However in order to provide a quick and flexible method of evaluating the geoid heights, this class evaluates the height by interpolation into a grid of precomputed values.

The height of the geoid above the ellipsoid, N, is sometimes called the geoid undulation. It can be used to convert a height above the ellipsoid, h, to the corresponding height above the geoid (the orthometric height, roughly the height above mean sea level), H, using the relations

h = N + H; H = −N + h.

See Geoid height for details of how to install the data sets, the data format, estimates of the interpolation errors, and how to use caching.

This class is typically not thread safe in that a single instantiation cannot be safely used by multiple threads because of the way the object reads the data set and because it maintains a single-cell cache. If multiple threads need to calculate geoid heights they should all construct thread-local instantiations. Alternatively, set the optional threadsafe parameter to true in the constructor. This causes the constructor to read all the data into memory and to turn off the single-cell caching which results in a Geoid object which is thread safe.

Example of use:

// Example of using the GeographicLib::Geoid class
// This requires that the egm96-5 geoid model be installed; see
// https://geographiclib.sourceforge.io/C++/doc/geoid.html#geoidinst
 
#include <iostream>
#include <exception>
#include <GeographicLib/Geoid.hpp>
 
using namespace std;
using namespace GeographicLib;
 
int main() {
  try {
    Geoid egm96("egm96-5");
    // Convert height above egm96 to height above the ellipsoid
    double lat = 42, lon = -75, height_above_geoid = 20;
    double
      geoid_height = egm96(lat, lon),
      height_above_ellipsoid = (height_above_geoid +
                                Geoid::GEOIDTOELLIPSOID * geoid_height);
    cout << height_above_ellipsoid << "\n";
  }
  catch (const exception& e) {
    cerr << "Caught exception: " << e.what() << "\n";
    return 1;
  }
}

GeoidEval is a command-line utility providing access to the functionality of Geoid.

Definition at line 82 of file Geoid.hpp.

Member Enumeration Documentation

◆ convertflag

enum GeographicLib::Geoid::convertflag

Flags indicating conversions between heights above the geoid and heights above the ellipsoid.

Enumerator
ELLIPSOIDTOGEOID	The multiplier for converting from heights above the ellipsoid to heights above the geoid.
NONE	No conversion.
GEOIDTOELLIPSOID	The multiplier for converting from heights above the geoid to heights above the ellipsoid.

Definition at line 178 of file Geoid.hpp.

Constructor & Destructor Documentation

◆ Geoid()

GeographicLib::Geoid::Geoid	(	const std::string &	name,
		const std::string &	path = `""`,
		bool	cubic = `true`,
		bool	threadsafe = `false`
	)

explicit

Construct a geoid.

Parameters

[in]	name	the name of the geoid.
[in]	path	(optional) directory for data file.
[in]	cubic	(optional) interpolation method; false means bilinear, true (the default) means cubic.
[in]	threadsafe	(optional), if true, construct a thread safe object. The default is false

Exceptions

GeographicErr	if the data file cannot be found, is unreadable, or is corrupt.
GeographicErr	if threadsafe is true but the memory necessary for caching the data can't be allocated.

The data file is formed by appending ".pgm" 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 DefaultGeoidPath(). If the threadsafe parameter is true, the data set is read into memory, the data file is closed, and single-cell caching is turned off; this results in a Geoid object which is thread safe.

Definition at line 201 of file Geoid.cpp.

References CacheAll(), DefaultGeoidPath(), GeographicLib::Math::hd, and GeographicLib::Math::td.

Member Function Documentation

◆ CacheArea()

void GeographicLib::Geoid::CacheArea	(	real	south,
		real	west,
		real	north,
		real	east
	)		const

Set up a cache.

Parameters

[in]	south	latitude (degrees) of the south edge of the cached area.
[in]	west	longitude (degrees) of the west edge of the cached area.
[in]	north	latitude (degrees) of the north edge of the cached area.
[in]	east	longitude (degrees) of the east edge of the cached area.

Exceptions

GeographicErr	if the memory necessary for caching the data can't be allocated (in this case, you will have no cache and can try again with a smaller area).
GeographicErr	if there's a problem reading the data.
GeographicErr	if this is called on a threadsafe Geoid.

Cache the data for the specified "rectangular" area bounded by the parallels south and north and the meridians west and east. east is always interpreted as being east of west, if necessary by adding 360° to its value. south and north should be in the range [−90°, 90°].

Definition at line 408 of file Geoid.cpp.

References GeographicLib::Math::AngNormalize(), CacheClear(), GeographicLib::Math::LatFix(), and GeographicLib::Math::td.

Referenced by main().

◆ CacheAll()

void GeographicLib::Geoid::CacheAll ( ) const

inline

Cache all the data.

Exceptions

GeographicErr	if the memory necessary for caching the data can't be allocated (in this case, you will have no cache and can try again with a smaller area).
GeographicErr	if there's a problem reading the data.
GeographicErr	if this is called on a threadsafe Geoid.

On most computers, this is fast for data sets with grid resolution of 5' or coarser. For a 1' grid, the required RAM is 450MB; a 2.5' grid needs 72MB; and a 5' grid needs 18MB.

Definition at line 258 of file Geoid.hpp.

References GeographicLib::Math::qd, and GeographicLib::Math::td.

Referenced by Geoid(), and main().

◆ CacheClear()

void GeographicLib::Geoid::CacheClear ( ) const

Clear the cache. This never throws an error. (This does nothing with a thread safe Geoid.)

Definition at line 395 of file Geoid.cpp.

References std::swap().

Referenced by CacheArea().

◆ operator()()

Math::real GeographicLib::Geoid::operator()	(	real	lat,
		real	lon
	)		const

inline

Compute the geoid height at a point

Parameters

[in]	lat	latitude of the point (degrees).
[in]	lon	longitude of the point (degrees).

Exceptions

GeographicErr if there's a problem reading the data; this never happens if (lat, lon) is within a successfully cached area.

Returns: the height of the geoid above the ellipsoid (meters).

The latitude should be in [−90°, 90°].

Definition at line 284 of file Geoid.hpp.

◆ ConvertHeight()

Math::real GeographicLib::Geoid::ConvertHeight	(	real	lat,
		real	lon,
		real	h,
		convertflag	d
	)		const

inline

Convert a height above the geoid to a height above the ellipsoid and vice versa.

Parameters

[in]	lat	latitude of the point (degrees).
[in]	lon	longitude of the point (degrees).
[in]	h	height of the point (degrees).
[in]	d	a Geoid::convertflag specifying the direction of the conversion; Geoid::GEOIDTOELLIPSOID means convert a height above the geoid to a height above the ellipsoid; Geoid::ELLIPSOIDTOGEOID means convert a height above the ellipsoid to a height above the geoid.

Exceptions

GeographicErr if there's a problem reading the data; this never happens if (lat, lon) is within a successfully cached area.

Returns: converted height (meters).

Definition at line 304 of file Geoid.hpp.

◆ Description()

const std::string & GeographicLib::Geoid::Description ( ) const

inline

Returns: geoid description, if available, in the data file; if absent, return "NONE".

Definition at line 318 of file Geoid.hpp.

Referenced by main().

◆ DateTime()

const std::string & GeographicLib::Geoid::DateTime ( ) const

inline

Returns: date of the data file; if absent, return "UNKNOWN".

Definition at line 323 of file Geoid.hpp.

Referenced by main().

◆ GeoidFile()

const std::string & GeographicLib::Geoid::GeoidFile ( ) const

inline

Returns: full file name used to load the geoid data.

Definition at line 328 of file Geoid.hpp.

Referenced by main().

◆ GeoidName()

const std::string & GeographicLib::Geoid::GeoidName ( ) const

inline

Returns: "name" used to load the geoid data (from the first argument of the constructor).

Definition at line 334 of file Geoid.hpp.

◆ GeoidDirectory()

const std::string & GeographicLib::Geoid::GeoidDirectory ( ) const

inline

Returns: directory used to load the geoid data.

Definition at line 339 of file Geoid.hpp.

◆ Interpolation()

const std::string GeographicLib::Geoid::Interpolation ( ) const

inline

Returns: interpolation method ("cubic" or "bilinear").

Definition at line 344 of file Geoid.hpp.

Referenced by main().

◆ MaxError()

Math::real GeographicLib::Geoid::MaxError ( ) const

inline

Returns: estimate of the maximum interpolation and quantization error (meters).

This relies on the value being stored in the data file. If the value is absent, return −1.

Definition at line 354 of file Geoid.hpp.

Referenced by main().

◆ RMSError()

Math::real GeographicLib::Geoid::RMSError ( ) const

inline

Returns: estimate of the RMS interpolation and quantization error (meters).

This relies on the value being stored in the data file. If the value is absent, return −1.

Definition at line 363 of file Geoid.hpp.

Referenced by main().

◆ Offset()

Math::real GeographicLib::Geoid::Offset ( ) const

inline

Returns: offset (meters).

This in used in converting from the pixel values in the data file to geoid heights.

Definition at line 371 of file Geoid.hpp.

Referenced by main().

◆ Scale()

Math::real GeographicLib::Geoid::Scale ( ) const

inline

Returns: scale (meters).

This in used in converting from the pixel values in the data file to geoid heights.

Definition at line 379 of file Geoid.hpp.

Referenced by main().

◆ ThreadSafe()

bool GeographicLib::Geoid::ThreadSafe ( ) const

inline

Returns: true if the object is constructed to be thread safe.

Definition at line 384 of file Geoid.hpp.

◆ Cache()

bool GeographicLib::Geoid::Cache ( ) const

inline

Returns: true if a data cache is active.

Definition at line 389 of file Geoid.hpp.

Referenced by main().

◆ CacheWest()

Math::real GeographicLib::Geoid::CacheWest ( ) const

inline

Returns: west edge of the cached area; the cache includes this edge.

Definition at line 394 of file Geoid.hpp.

Referenced by main().

◆ CacheEast()

Math::real GeographicLib::Geoid::CacheEast ( ) const

inline

Returns: east edge of the cached area; the cache excludes this edge.

Definition at line 403 of file Geoid.hpp.

Referenced by main().

◆ CacheNorth()

Math::real GeographicLib::Geoid::CacheNorth ( ) const

inline

Returns: north edge of the cached area; the cache includes this edge.

Definition at line 413 of file Geoid.hpp.

References GeographicLib::Math::qd.

Referenced by main().

◆ CacheSouth()

Math::real GeographicLib::Geoid::CacheSouth ( ) const

inline

Returns: south edge of the cached area; the cache excludes this edge unless it's the south pole.

Definition at line 421 of file Geoid.hpp.

References GeographicLib::Math::qd.

Referenced by main().

◆ EquatorialRadius()

Math::real GeographicLib::Geoid::EquatorialRadius ( ) const

inline

Returns: a the equatorial radius of the WGS84 ellipsoid (meters).

(The WGS84 value is returned because the supported geoid models are all based on this ellipsoid.)

Definition at line 433 of file Geoid.hpp.

References GeographicLib::Constants::WGS84_a().

◆ Flattening()

Math::real GeographicLib::Geoid::Flattening ( ) const

inline

Returns: f the flattening of the WGS84 ellipsoid.

(The WGS84 value is returned because the supported geoid models are all based on this ellipsoid.)

Definition at line 442 of file Geoid.hpp.

References GeographicLib::Constants::WGS84_f().

◆ DefaultGeoidPath()

string GeographicLib::Geoid::DefaultGeoidPath ( )

static

Returns: the default path for geoid data files.

This is the value of the environment variable GEOGRAPHICLIB_GEOID_PATH, if set; otherwise, it is $GEOGRAPHICLIB_DATA/geoids if the environment variable GEOGRAPHICLIB_DATA is set; otherwise, it is a compile-time default (/usr/local/share/GeographicLib/geoids on non-Windows systems and C:/ProgramData/GeographicLib/geoids on Windows systems).

Definition at line 489 of file Geoid.cpp.

References GEOGRAPHICLIB_DATA.

Referenced by Geoid().

◆ DefaultGeoidName()

string GeographicLib::Geoid::DefaultGeoidName ( )

static

Returns: the default name for the geoid.

This is the value of the environment variable GEOGRAPHICLIB_GEOID_NAME, if set; otherwise, it is "egm96-5". The Geoid class does not use this function; it is just provided as a convenience for a calling program when constructing a Geoid object.

Definition at line 502 of file Geoid.cpp.

References GEOGRAPHICLIB_GEOID_DEFAULT_NAME.

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

Public Types

Public Member Functions

Static Public Member Functions

Detailed Description

Member Enumeration Documentation

◆ convertflag

Constructor & Destructor Documentation

◆ Geoid()

Member Function Documentation

◆ CacheArea()

◆ CacheAll()

◆ CacheClear()

◆ operator()()

◆ ConvertHeight()

◆ Description()

◆ DateTime()

◆ GeoidFile()

◆ GeoidName()

◆ GeoidDirectory()

◆ Interpolation()

◆ MaxError()

◆ RMSError()

◆ Offset()

◆ Scale()

◆ ThreadSafe()

◆ Cache()

◆ CacheWest()

◆ CacheEast()

◆ CacheNorth()

◆ CacheSouth()

◆ EquatorialRadius()

◆ Flattening()

◆ DefaultGeoidPath()

◆ DefaultGeoidName()