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# NAME

Geo::Calc::XS - simple geo calculator for points and distances

# SYNOPSIS

`````` use Geo::Calc::XS;

my \$gc            = Geo::Calc::XS->new( lat => 40.417875, lon => -3.710205 );
my \$lan           = \$gc->get_lan();
my \$lon           = \$gc->get_lon();
my \$units         = \$gc->get_units();
my \$distance      = \$gc->distance_to( { lat => 40.422371, lon => -3.704298 }, -6 );
my \$brng          = \$gc->bearing_to( { lat => 40.422371, lon => -3.704298 }, -6 );
my \$f_brng        = \$gc->final_bearing_to( { lat => 40.422371, lon => -3.704298 }, -6 );
my \$midpoint      = \$gc->midpoint_to( { lat => 40.422371, lon => -3.704298 }, -6 );
my \$destination   = \$gc->destination_point( 90, 1, -6 );
my \$bbox          = \$gc->boundry_box( 3, 4, -6 );
my \$r_distance    = \$gc->rhumb_distance_to( { lat => 40.422371, lon => -3.704298 }, -6 );
my \$r_brng        = \$gc->rhumb_bearing_to( { lat => 40.422371, lon => -3.704298 }, -6 );
my \$r_destination = \$gc->rhumb_destination_point( 30, 1, -6 );
my \$point         = \$gc->intersection( 90, { lat => 40.422371, lon => -3.704298 }, 180, -6 );``````

# DESCRIPTION

Geo::Calc::XS implements a variety of calculations for latitude/longitude points

All these formulas are for calculations on the basis of a spherical earth (ignoring ellipsoidal effects), which is accurate enough for most purposes.

[ In fact, the earth is very slightly ellipsoidal; using a spherical model gives errors typically up to 0.3% ].

Benchmarking this module and Geo::Calc I found out that this module is sometimes more than 8000 times faster.

This module is designed to be thread-safe, although, of course, interpreter-based threads are officially discouraged (see http://perldoc.perl.org/threads.html).

# CAVEATS

This is not a drop-in replacement for Geo::Calc, see the COMPATIBILITY section further down.

# Geo::Calc::XS->new()

`````` \$gc = Geo::Calc::XS->new( lat => 40.417875, lon => -3.710205 ); # Somewhere in Madrid
\$gc = Geo::Calc::XS->new( lat => 51.503269, lon => 0, units => 'k-m' ); # The O2 Arena in London``````

Creates a new Geo::Calc::XS object from a latitude and longitude. The default decimal precision is -6 for all functions => meaning by default it always returns the results with 6 decimals.

The default unit distance is 'm' (meter), but you cand define another unit using `units`. Accepted values are: 'm' (meters), 'k-m' (kilometers), 'yd' (yards), 'ft' (feet) and 'mi' (miles)

If a `radius` parameter is passed, it is ignored.

Returns a reference to a `Geo::Calc::XS` object.

## Parameters

Each of these parameters can be accessed after construction using `get_lat`, `get_lon`, `get_radius` or `get_units`.

lat

=> latitude of the point ( required )

lon

=> longitude of the point ( required )

=> earth radius in km ( defaults to 6371 )

units

=> the distance unit received and output by this object ( defaults to 'm' )

# METHODS

## distance_to

`````` \$gc->distance_to( \$point[, \$precision] )
\$gc->distance_to( { lat => 40.422371, lon => -3.704298 } )
\$gc->distance_to( Geo::Calc::XS->new( lat => 40.422371, lon => -3.704298 ) )``````

This uses the "haversine" formula to calculate great-circle distances between the two points - that is, the shortest distance over the earth's surface - giving an `as-the-crow-flies` distance between the points (ignoring any hills!)

The haversine formula `remains particularly well-conditioned for numerical computation even at small distances` - unlike calculations based on the spherical law of cosines. It was published by R W Sinnott in Sky and Telescope, 1984, though known about for much longer by navigators. (For the curious, c is the angular distance in radians, and a is the square of half the chord length between the points).

Returns with the distance using the precision defined or -6 ( -6 = 6 decimals ( eg 4.000001 ) ), in this object's distance unit.

## bearing_to

`````` \$gc->bearing_to( \$point[, \$precision] );
\$gc->bearing_to( { lat => 40.422371, lon => -3.704298 }, -6 );
\$gc->bearing_to( Geo::Calc::XS->new( lat => 40.422371, lon => -3.704298 ), -6 );``````

In general, your current heading will vary as you follow a great circle path (orthodrome); the final heading will differ from the initial heading by varying degrees according to distance and latitude (if you were to go from say 35N,45E (Baghdad) to 35N,135E (Osaka), you would start on a heading of 60 and end up on a heading of 120!).

This formula is for the initial bearing (sometimes referred to as forward azimuth) which if followed in a straight line along a great-circle arc will take you from the start point to the end point

Returns the (initial) bearing from this point to the supplied point, in degrees with the specified pricision

## final_bearing_to

`````` my \$f_brng = \$gc->final_bearing_to( \$point[, \$precision] );
my \$f_brng = \$gc->final_bearing_to( { lat => 40.422371, lon => -3.704298 } );
my \$f_brng = \$gc->final_bearing_to( Geo::Calc::XS->new( lat => 40.422371, lon => -3.704298 ) );``````

Returns final bearing (in degrees) arriving at supplied destination point from this point; the final bearing will differ from the initial bearing by varying degrees according to distance and latitude

## midpoint_to

`````` \$gc->midpoint_to( \$point[, \$precision] );
\$gc->midpoint_to( { lat => 40.422371, lon => -3.704298 } );
\$gc->midpoint_to( Geo::Calc::XS->new( lat => 40.422371, lon => -3.704298 ) );``````

Returns the midpoint along a great circle path between the initial point and the supplied point.

See http://mathforum.org/library/drmath/view/51822.html for derivation

## destination_point

`````` \$gc->destination_point( \$bearing, \$distance[, \$precision] );
\$gc->destination_point( 90, 1 );``````

Returns the destination point and the final bearing using Vincenty inverse formula for ellipsoids.

`\$bearing` must be specified in degrees, where 0 is north and 90 is east, and `\$distance` must be specified in this object's distance unit.

## boundry_box

`````` \$gc->boundry_box( \$width[, \$height[, \$precision]] );
\$gc->boundry_box( 3, 4 ); # will generate a 3x4m box around the point, assuming the object's distance unit is meters
\$gc->boundry_box( 1 ); # will generate a 2x2m box around the point (radius), assuming the object's distance unit is meters``````

Returns the boundry box min/max having the initial point defined as the center of the boundry box, given the width and height.

If only one dimension has been specified, than that dimension is considered a radius.

Dimensions should be specified in the object's distance unit.

## rhumb_distance_to

`````` \$gc->rhumb_distance_to( \$point[, \$precision] );
\$gc->rhumb_distance_to( { lat => 40.422371, lon => -3.704298 } );
\$gc->rhumb_distance_to( Geo::Calc::XS->new( lat => 40.422371, lon => -3.704298 ) );``````

Returns the distance from this point to the supplied point, in the object's distance unit, travelling along a rhumb line.

A 'rhumb line' (or loxodrome) is a path of constant bearing, which crosses all meridians at the same angle.

Sailors used to (and sometimes still) navigate along rhumb lines since it is easier to follow a constant compass bearing than to be continually adjusting the bearing, as is needed to follow a great circle. Rhumb lines are straight lines on a Mercator Projection map (also helpful for navigation).

Rhumb lines are generally longer than great-circle (orthodrome) routes. For instance, London to New York is 4% longer along a rhumb line than along a great circle . important for aviation fuel, but not particularly to sailing vessels. New York to Beijing . close to the most extreme example possible (though not sailable!) . is 30% longer along a rhumb line.

## rhumb_bearing_to

`````` \$gc->rhumb_bearing_to( \$point[, \$precision] );
\$gc->rhumb_bearing_to( { lat => 40.422371, lon => -3.704298 } );
\$gc->rhumb_bearing_to( Geo::Calc::XS->new( lat => 40.422371, lon => -3.704298 ) );``````

Returns the bearing from this point to the supplied point along a rhumb line, in degrees

## rhumb_destination_point

`````` \$gc->rhumb_destination_point( \$brng, \$distance[, \$precision] );
\$gc->rhumb_destination_point( 30, 1 );``````

Returns the destination point from this point having travelled the given distance (in the object's distance unit) on the given bearing along a rhumb line.

## intersection

`````` \$gc->intersection( \$brng1, \$point, \$brng2[, \$precision] );
\$gc->intersection( 90, { lat => 40.422371, lon => -3.704298 }, 180 );
\$gc->intersection( 90, Geo::Calc::XS->new( lat => 40.422371, lon => -3.704298 ), 180 );``````

Returns the point of intersection of two paths defined by point and bearing

## distance_at

Returns the distance in meters for 1deg of latitude and longitude at the specified latitude.

`````` my \$m_distance = \$self->distance_at([\$precision]);
my \$m_distance = \$self->distance_at();
# at lat 2 with precision -6 returns { m_lat => 110575.625009, m_lon => 111252.098718 }``````

Note that this method always returns distances in meters, unlike all the other methods which use the object's distance unit. This is kept as it is for backwards compatibility.

# COMPATIBILITY

A Geo::Calc::XS object does not have the same interface as a Geo::Calc object, despite the similarities.

Here are the currently known differences:

• `destination_point_hs` is provided by Geo::Calc but not by this module.

• The constructor for Geo::Calc accepts a `radius` parameter, but this module ignores it.

• Methods with identicial names perform similar functions but may return different results after a few decimal places.

• It's undocumented whether Geo::Calc is thread-safe, whereas this module does intend to be thread-safe.

Geo::Calc is the original implementation, which is abandoned at time of writing (Aug 2014).

# REPOSITORY

https://github.com/Flimm/perl5-geo-calc-xs

# BUGS

All complex software has bugs lurking in it, and this module is no exception.

Please report any bugs through the web interface at https://rt.cpan.org/Public/Dist/Display.html?Name=Geo-Calc-XS.

# AUTHOR

Sorin Alexandru Pop `<asp@cpan.org>`

# THANKS

Marius Crisan `<crisan.marius@gmail.com>`

David D Lowe `<flimm@cpan.org>`

Chris Hughes `<chris@lokku.com>`