GeographicLib::OSGB Class Reference

Ordnance Survey grid system for Great Britain. More...

#include <OSGB.hpp>

Static Public Member Functions
static void	Forward (real lat, real lon, real &x, real &y)
static void	Forward (real lat, real lon, real &x, real &y, real &gamma, real &k)
static void	GridReference (const std::string &gridref, real &x, real &y, int &prec, bool centerp=true)
static void	GridReference (real x, real y, int prec, std::string &gridref)
static void	Reverse (real x, real y, real &lat, real &lon)
static void	Reverse (real x, real y, real &lat, real &lon, real &gamma, real &k)
Inspector functions
static Math::real	MajorRadius ()
static Math::real	Flattening ()
static Math::real	CentralScale ()
static Math::real	OriginLatitude ()
static Math::real	OriginLongitude ()
static Math::real	FalseNorthing ()
static Math::real	FalseEasting ()

Private Types
enum	{   base_ = 10, tile_ = 100000, tilelevel_ = 5, tilegrid_ = 5,   tileoffx_ = 2 * tilegrid_, tileoffy_ = 1 * tilegrid_, minx_ = - tileoffx_ * tile_, miny_ = - tileoffy_ * tile_,   maxx_ = (tilegrid_*tilegrid_ - tileoffx_) * tile_, maxy_ = (tilegrid_*tilegrid_ - tileoffy_) * tile_, maxprec_ = 5 + 6 }
typedef Math::real	real

Private Member Functions
	OSGB ()

Static Private Member Functions
static void	CheckCoords (real x, real y)
static real	computenorthoffset ()
static const TransverseMercator &	OSGBTM ()

Static Private Attributes
static const char *const	digits_ = "0123456789"
static const char *const	letters_ = "ABCDEFGHJKLMNOPQRSTUVWXYZ"

Detailed Description

Ordnance Survey grid system for Great Britain.

The class implements the coordinate system used by the Ordnance Survey for maps of Great Britain and conversions to the grid reference system.

See

• A guide to coordinate systems in Great Britain
• Guide to the National Grid

Warning

the latitudes and longitudes for the Ordnance Survey grid system do not use the WGS84 datum. Do not use the values returned by this class in the UTMUPS, MGRS, or Geoid classes without first converting the datum (and vice versa).

Example of use:

// Example of using the GeographicLib::OSGB class
 
#include <iostream>
#include <iomanip>
#include <exception>
#include <string>
#include <GeographicLib/OSGB.hpp>
#include <GeographicLib/DMS.hpp>
 
using namespace std;
using namespace GeographicLib;
 
int main() {
  try {
    {
      // Sample forward calculation from
      // A guide to coordinate systems in Great Britain
      double
        lat = DMS::Decode(52,39,27.2531),
        lon = DMS::Decode( 1,43, 4.5177);
      double x, y;
      OSGB::Forward(lat, lon, x, y);
      string gridref;
      OSGB::GridReference(x, y, 2, gridref);
      cout << fixed << setprecision(3)
           << x << " " << y << " " << gridref << "\n";
    }
    {
      // Sample reverse calculation
      string gridref = "TG5113";
      double x, y;
      int prec;
      OSGB::GridReference(gridref, x, y, prec);
      double lat, lon;
      OSGB::Reverse(x, y, lat, lon);
      cout << fixed << setprecision(8)
           << prec << " " << lat << " " << lon << "\n";
    }
  }
  catch (const exception& e) {
    cerr << "Caught exception: " << e.what() << "\n";
    return 1;
  }
}

Definition at line 44 of file OSGB.hpp.

Member Typedef Documentation

real

typedef Math::real GeographicLib::OSGB::real

private

Definition at line 46 of file OSGB.hpp.

Member Enumeration Documentation

anonymous enum

anonymous enum

private

Enumerator
base_
tile_
tilelevel_
tilegrid_
tileoffx_
tileoffy_
minx_
miny_
maxx_
maxy_
maxprec_

Definition at line 50 of file OSGB.hpp.

Constructor & Destructor Documentation

OSGB()

GeographicLib::OSGB::OSGB ( )

private

Member Function Documentation

CentralScale()

static Math::real GeographicLib::OSGB::CentralScale ( )

inlinestatic

Returns

k0 central scale for the OSGB projection (0.9996012717...).

C. J. Mugnier, Grids & Datums, PE&RS, Oct. 2003, states that this is defined as 10^{9.9998268−10}.

Definition at line 214 of file OSGB.hpp.

CheckCoords()

void GeographicLib::OSGB::CheckCoords ( real  x, real  y  )

staticprivate

Definition at line 152 of file src/OSGB.cpp.

computenorthoffset()

Math::real GeographicLib::OSGB::computenorthoffset ( )

staticprivate

Definition at line 26 of file src/OSGB.cpp.

FalseEasting()

static Math::real GeographicLib::OSGB::FalseEasting ( )

inlinestatic

Returns

false easting the OSGB projection (400000 meters).

Definition at line 238 of file OSGB.hpp.

FalseNorthing()

static Math::real GeographicLib::OSGB::FalseNorthing ( )

inlinestatic

Returns

false northing the OSGB projection (−100000 meters).

Definition at line 233 of file OSGB.hpp.

Flattening()

static Math::real GeographicLib::OSGB::Flattening ( )

inlinestatic

Returns

f the inverse flattening of the Airy 1830 ellipsoid.

For the Airy 1830 ellipsoid, a = 20923713 ft and b = 20853810 ft; thus the flattening = (20923713 − 20853810)/20923713 = 7767/2324857 = 1/299.32496459... (The Airy 1830 value is returned because the OSGB projection is based on this ellipsoid.)

Definition at line 205 of file OSGB.hpp.

Forward() [1/2]

static void GeographicLib::OSGB::Forward ( real  lat, real  lon, real &  x, real &  y  )

inlinestatic

OSGB::Forward without returning the convergence and scale.

Definition at line 112 of file OSGB.hpp.

Forward() [2/2]

static void GeographicLib::OSGB::Forward ( real  lat, real  lon, real &  x, real &  y, real &  gamma, real &  k  )

inlinestatic

Forward projection, from geographic to OSGB coordinates.

Parameters

[in]	lat	latitude of point (degrees).
[in]	lon	longitude of point (degrees).
[out]	x	easting of point (meters).
[out]	y	northing of point (meters).
[out]	gamma	meridian convergence at point (degrees).
[out]	k	scale of projection at point.

lat should be in the range [−90°, 90°].

Definition at line 81 of file OSGB.hpp.

GridReference() [1/2]

void GeographicLib::OSGB::GridReference ( const std::string &  gridref, real &  x, real &  y, int &  prec, bool  centerp = true  )

static

Convert OSGB coordinates to a grid reference.

Parameters

[in]	gridref	National Grid reference.
[out]	x	easting of point (meters).
[out]	y	northing of point (meters).
[out]	prec	precision relative to 100 km.
[in]	centerp	if true (default), return center of the grid square, else return SW (lower left) corner.

Exceptions

GeographicErr

if gridref is illegal.

The grid reference must be of the form: two letters (not including I) followed by an even number of digits (up to 22).

If the first 2 characters of gridref are "IN", then x and y are set to NaN and prec is set to −2.

Definition at line 87 of file src/OSGB.cpp.

GridReference() [2/2]

void GeographicLib::OSGB::GridReference ( real  x, real  y, int  prec, std::string &  gridref  )

static

Convert OSGB coordinates to a grid reference.

Parameters

[in]	x	easting of point (meters).
[in]	y	northing of point (meters).
[in]	prec	precision relative to 100 km.
[out]	gridref	National Grid reference.

Exceptions

GeographicErr	if prec, x, or y is outside its allowed range.
std::bad_alloc	if the memory for gridref can't be allocatied.

prec specifies the precision of the grid reference string as follows:

• prec = 0 (min), 100km
• prec = 1, 10km
• prec = 2, 1km
• prec = 3, 100m
• prec = 4, 10m
• prec = 5, 1m
• prec = 6, 0.1m
• prec = 11 (max), 1μm

The easting must be in the range [−1000 km, 1500 km) and the northing must be in the range [−500 km, 2000 km). These bounds are consistent with rules for the letter designations for the grid system.

If x or y is NaN, the returned grid reference is "INVALID".

Definition at line 34 of file src/OSGB.cpp.

MajorRadius()

static Math::real GeographicLib::OSGB::MajorRadius ( )

inlinestatic

Returns

a the equatorial radius of the Airy 1830 ellipsoid (meters).

This is 20923713 ft converted to meters using the rule 1 ft = 10^{9.48401603−10} m. The Airy 1830 value is returned because the OSGB projection is based on this ellipsoid. The conversion factor from feet to meters is the one used for the 1936 retriangulation of Britain; see Section A.1 (p. 37) of A guide to coordinate systems in Great Britain, v2.2 (Dec. 2013).

Definition at line 190 of file OSGB.hpp.

OriginLatitude()

static Math::real GeographicLib::OSGB::OriginLatitude ( )

inlinestatic

Returns

latitude of the origin for the OSGB projection (49 degrees).

Definition at line 222 of file OSGB.hpp.

OriginLongitude()

static Math::real GeographicLib::OSGB::OriginLongitude ( )

inlinestatic

Returns

longitude of the origin for the OSGB projection (−2 degrees).

Definition at line 228 of file OSGB.hpp.

OSGBTM()

const TransverseMercator & GeographicLib::OSGB::OSGBTM ( )

staticprivate

Definition at line 20 of file src/OSGB.cpp.

Reverse() [1/2]

static void GeographicLib::OSGB::Reverse ( real  x, real  y, real &  lat, real &  lon  )

inlinestatic

OSGB::Reverse without returning the convergence and scale.

Definition at line 120 of file OSGB.hpp.

Reverse() [2/2]

static void GeographicLib::OSGB::Reverse ( real  x, real  y, real &  lat, real &  lon, real &  gamma, real &  k  )

inlinestatic

Reverse projection, from OSGB coordinates to geographic.

Parameters

[in]	x	easting of point (meters).
[in]	y	northing of point (meters).
[out]	lat	latitude of point (degrees).
[out]	lon	longitude of point (degrees).
[out]	gamma	meridian convergence at point (degrees).
[out]	k	scale of projection at point.

The value of lon returned is in the range [−180°, 180°].

Definition at line 102 of file OSGB.hpp.

Member Data Documentation

digits_

const char *const GeographicLib::OSGB::digits_ = "0123456789"

staticprivate

Definition at line 48 of file OSGB.hpp.

letters_

const char *const GeographicLib::OSGB::letters_ = "ABCDEFGHJKLMNOPQRSTUVWXYZ"

staticprivate

Definition at line 47 of file OSGB.hpp.

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

• OSGB.hpp
• src/OSGB.cpp