GeographicLib::Geohash Class Reference

Conversions for geohashes. More...

#include <GeographicLib/Geohash.hpp>

Static Public Member Functions
static void	Forward (real lat, real lon, int len, std::string &geohash)
static void	Reverse (const std::string &geohash, real &lat, real &lon, int &len, bool centerp=true)
static Math::real	LatitudeResolution (int len)
static Math::real	LongitudeResolution (int len)
static int	GeohashLength (real res)
static int	GeohashLength (real latres, real lonres)
static int	DecimalPrecision (int len)

Detailed Description

Conversions for geohashes.

Geohashes are described in

• https://en.wikipedia.org/wiki/Geohash
• http://geohash.org/

They provide a compact string representation of a particular geographic location (expressed as latitude and longitude), with the property that if trailing characters are dropped from the string the geographic location remains nearby. The classes Georef and GARS implement similar compact representations.

Example of use:

// Example of using the GeographicLib::Geohash class
 
#include <iostream>
#include <iomanip>
#include <exception>
#include <string>
#include <GeographicLib/Geohash.hpp>
 
using namespace std;
using namespace GeographicLib;
 
int main() {
  try {
    {
      // Sample forward calculation
      double lat = 57.64911, lon = 10.40744; // Jutland (the wikipedia example)
      string geohash;
      int maxlen = Geohash::GeohashLength(1.0e-5);
      for (int len = 0; len <= maxlen; ++len) {
        Geohash::Forward(lat, lon, len, geohash);
        cout << len << " " << geohash << "\n";
      }
    }
    {
      // Sample reverse calculation
      string geohash = "u4pruydqqvj";
      double lat, lon, latres, lonres;
      cout << fixed;
      for (unsigned i = 0; i <= geohash.length(); ++i) {
        int len;
        Geohash::Reverse(geohash.substr(0, i), lat, lon, len);
        latres = Geohash::LatitudeResolution(len);
        lonres = Geohash::LongitudeResolution(len);
        cout << setprecision(max(0, Geohash::DecimalPrecision(len)))
             << len << " "
             << lat << "+/-" << latres/2 << " "
             << lon << "+/-" << lonres/2 << "\n";
      }
    }
  }
  catch (const exception& e) {
    cerr << "Caught exception: " << e.what() << "\n";
    return 1;
  }
}

Definition at line 40 of file Geohash.hpp.

Member Function Documentation

Forward()

void GeographicLib::Geohash::Forward ( real  lat, real  lon, int  len, std::string &  geohash  )

static

Convert from geographic coordinates to a geohash.

Parameters

[in]	lat	latitude of point (degrees).
[in]	lon	longitude of point (degrees).
[in]	len	the length of the resulting geohash.
[out]	geohash	the geohash.

Exceptions

GeographicErr	if lat is not in [−90°, 90°].
std::bad_alloc	if memory for geohash can't be allocated.

Internally, len is first put in the range [0, 18]. (len = 18 provides approximately 1μm precision.)

If lat or lon is NaN, the returned geohash is "invalid".

Definition at line 25 of file Geohash.cpp.

References GeographicLib::Math::AngNormalize(), GeographicLib::Math::hd, GeographicLib::Math::qd, and GeographicLib::Utility::str().

Reverse()

void GeographicLib::Geohash::Reverse ( const std::string &  geohash, real &  lat, real &  lon, int &  len, bool  centerp = true  )

static

Convert from a geohash to geographic coordinates.

Parameters

[in]	geohash	the geohash.
[out]	lat	latitude of point (degrees).
[out]	lon	longitude of point (degrees).
[out]	len	the length of the geohash.
[in]	centerp	if true (the default) return the center of the geohash location, otherwise return the south-west corner.

Exceptions

GeographicErr

if geohash contains illegal characters.

Only the first 18 characters for geohash are considered. (18 characters provides approximately 1μm precision.) The case of the letters in geohash is ignored.

If the first 3 characters of geohash are "inv", then lat and lon are set to NaN and len is unchanged. ("nan" is treated similarly.)

Definition at line 67 of file Geohash.cpp.

References GeographicLib::Math::hd, GeographicLib::Utility::lookup(), GeographicLib::Math::NaN(), and GeographicLib::Math::qd.

LatitudeResolution()

static Math::real GeographicLib::Geohash::LatitudeResolution ( int  len )

inlinestatic

The latitude resolution of a geohash.

Parameters

[in]

len

the length of the geohash.

Returns

the latitude resolution (degrees).

Internally, len is first put in the range [0, 18].

Definition at line 99 of file Geohash.hpp.

References GeographicLib::Math::hd.

LongitudeResolution()

static Math::real GeographicLib::Geohash::LongitudeResolution ( int  len )

inlinestatic

The longitude resolution of a geohash.

Parameters

[in]

len

the length of the geohash.

Returns

the longitude resolution (degrees).

Internally, len is first put in the range [0, 18].

Definition at line 113 of file Geohash.hpp.

References GeographicLib::Math::td.

GeohashLength() [1/2]

static int GeographicLib::Geohash::GeohashLength ( real  res )

inlinestatic

The geohash length required to meet a given geographic resolution.

Parameters

[in]

res

the minimum of resolution in latitude and longitude (degrees).

Returns

geohash length.

The returned length is in the range [0, 18].

Definition at line 128 of file Geohash.hpp.

GeohashLength() [2/2]

static int GeographicLib::Geohash::GeohashLength ( real  latres, real  lonres  )

inlinestatic

The geohash length required to meet a given geographic resolution.

Parameters

[in]	latres	the resolution in latitude (degrees).
[in]	lonres	the resolution in longitude (degrees).

Returns

geohash length.

The returned length is in the range [0, 18].

Definition at line 145 of file Geohash.hpp.

DecimalPrecision()

static int GeographicLib::Geohash::DecimalPrecision ( int  len )

inlinestatic

The decimal geographic precision required to match a given geohash length. This is the number of digits needed after decimal point in a decimal degrees representation.

Parameters

[in]

len

the length of the geohash.

Returns

the decimal precision (may be negative).

Internally, len is first put in the range [0, 18]. The returned decimal precision is in the range [−2, 12].

Definition at line 167 of file Geohash.hpp.

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

• Geohash.hpp
• Geohash.cpp