GeographicLib  1.43
GeographicLib::Geoid Class Reference

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 Cache () const

Math::real CacheWest () const

Math::real CacheEast () const

Math::real CacheNorth () const

Math::real CacheSouth () 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.

In addition to returning the geoid height, the gradient of the geoid can be calculated. The gradient is defined as the rate of change of the geoid as a function of position on the ellipsoid. This uses the parameters for the WGS84 ellipsoid. The gradient defined in terms of the interpolated heights. As a result of the way that the geoid data is stored, the calculation of gradients can result in large quantization errors. This is particularly acute for fine grids, at high latitudes, and for the easterly gradient. For this reason, the use of this facility is DEPRECATED. Instead, use the GravityModel class to evaluate the gravity vector.

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
#include <iostream>
#include <exception>
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;
}
return 0;
}

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

Definition at line 92 of file Geoid.hpp.

## Member Enumeration Documentation

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

Enumerator
ELLIPSOIDTOGEOID

The multiplier for converting from heights above the geoid to heights above the ellipsoid.

NONE

No conversion.

GEOIDTOELLIPSOID

The multiplier for converting from heights above the ellipsoid to heights above the geoid.

Definition at line 193 of file Geoid.hpp.

## Constructor & Destructor Documentation

 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 200 of file Geoid.cpp.

References CacheAll(), and DefaultGeoidPath().

## Member Function Documentation

 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°]; west and east should be in the range [−540°, 540°).

Definition at line 442 of file Geoid.cpp.

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

Referenced by main().

 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 272 of file Geoid.hpp.

Referenced by Geoid(), and main().

 void GeographicLib::Geoid::CacheClear ( ) const

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

Definition at line 429 of file Geoid.cpp.

Referenced by CacheArea().

 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°] and longitude should be in [−540°, 540°).

Definition at line 298 of file Geoid.hpp.

 Math::real GeographicLib::Geoid::operator() ( real lat, real lon, real & gradn, real & grade ) const
inline

Compute the geoid height and gradient at a point

Parameters
 [in] lat latitude of the point (degrees). [in] lon longitude of the point (degrees). [out] gradn northerly gradient (dimensionless). [out] grade easterly gradient (dimensionless).
Exceptions
 GeographicErr if there's a problem reading the data; this never happens if (lat, lon) is within a successfully cached area.
Returns
geoid height (meters).

The latitude should be in [−90°, 90°] and longitude should be in [−540°, 540°). As a result of the way that the geoid data is stored, the calculation of gradients can result in large quantization errors. This is particularly acute for fine grids, at high latitudes, and for the easterly gradient. For this reason, the computation of the gradient is DEPRECATED. If you need to compute the direction of the acceleration due to gravity accurately, you should use GravityModel::Gravity.

Definition at line 323 of file Geoid.hpp.

 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 342 of file Geoid.hpp.

 const std::string& GeographicLib::Geoid::Description ( ) const
inline
Returns
geoid description, if available, in the data file; if absent, return "NONE".

Definition at line 357 of file Geoid.hpp.

Referenced by main().

 const std::string& GeographicLib::Geoid::DateTime ( ) const
inline
Returns
date of the data file; if absent, return "UNKNOWN".

Definition at line 362 of file Geoid.hpp.

Referenced by main().

 const std::string& GeographicLib::Geoid::GeoidFile ( ) const
inline
Returns
full file name used to load the geoid data.

Definition at line 367 of file Geoid.hpp.

Referenced by main().

 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 373 of file Geoid.hpp.

 const std::string& GeographicLib::Geoid::GeoidDirectory ( ) const
inline
Returns
directory used to load the geoid data.

Definition at line 378 of file Geoid.hpp.

 const std::string GeographicLib::Geoid::Interpolation ( ) const
inline
Returns
interpolation method ("cubic" or "bilinear").

Definition at line 383 of file Geoid.hpp.

Referenced by main().

 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 393 of file Geoid.hpp.

Referenced by main().

 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 402 of file Geoid.hpp.

Referenced by main().

 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 410 of file Geoid.hpp.

Referenced by main().

 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 418 of file Geoid.hpp.

Referenced by main().

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

Definition at line 423 of file Geoid.hpp.

 bool GeographicLib::Geoid::Cache ( ) const
inline
Returns
true if a data cache is active.

Definition at line 428 of file Geoid.hpp.

Referenced by main().

 Math::real GeographicLib::Geoid::CacheWest ( ) const
inline
Returns
west edge of the cached area; the cache includes this edge.

Definition at line 433 of file Geoid.hpp.

Referenced by main().

 Math::real GeographicLib::Geoid::CacheEast ( ) const
inline
Returns
east edge of the cached area; the cache excludes this edge.

Definition at line 442 of file Geoid.hpp.

Referenced by main().

 Math::real GeographicLib::Geoid::CacheNorth ( ) const
inline
Returns
north edge of the cached area; the cache includes this edge.

Definition at line 452 of file Geoid.hpp.

Referenced by main().

 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 460 of file Geoid.hpp.

Referenced by main().

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 470 of file Geoid.hpp.

References GeographicLib::Constants::WGS84_a().

 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 479 of file Geoid.hpp.

References GeographicLib::Constants::WGS84_f().

 std::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 521 of file Geoid.cpp.

References GEOGRAPHICLIB_DATA.

Referenced by Geoid(), and main().

 std::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 534 of file Geoid.cpp.

References GEOGRAPHICLIB_GEOID_DEFAULT_NAME.

Referenced by main().

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