NAME
Geo::Distance - Calculate Distances and Closest Locations
SYNOPSIS
use Geo::Distance;
my $geo = new Geo::Distance;
$geo->formula('hsin');
$geo->reg_unit( 'toad_hop', 200120 );
$geo->reg_unit( 'frog_hop' => 6 => 'toad_hop' );
my $distance = $geo->distance( 'unit_type', $lon1,$lat1 => $lon2,$lat2 );
my $locations = $geo->closest(
dbh => $dbh,
table => $table,
lon => $lon,
lat => $lat,
unit => $unit_type,
distance => $dist_in_unit
);
DESCRIPTION
This perl library aims to provide as many tools to make it as simple as
possible to calculate distances between geographic points, and anything
that can be derived from that. Currently there is support for finding
the closest locations within a specified distance, to find the closest
number of points to a specified point, and to do basic point-to-point
distance calculations.
DECOMMISSIONED
The GIS::Distance module is being worked on as a replacement for this
module. In the near future Geo::Distance will become a lightweight
wrapper around GIS::Distance so that legacy code benefits from fixes to
GIS::Distance through the old Geo::Distance API. For any new
developement I suggest that you look in to GIS::Distance.
STABILITY
The interface to Geo::Distance is fairly stable nowadays. If this
changes it will be noted here.
0.10 - The closest() method has a changed argument syntax and no longer
supports array searches. 0.09 - Changed the behavior of the reg_unit
funtcion. 0.07 - OO only, and other changes all over.
PROPERTIES
UNITS
All functions accept a unit type to do the computations of distance
with. By default no units are defined in a Geo::Distance object. You can
add units with reg_unit() or create some default units with
default_units().
LATITUDE AND LONGITUDE
When a function needs a lon and lat they must always be in decimal
degree format. Here is some sample code for converting from other
formats to decimal:
# DMS to Decimal
my $decimal = $degrees + ($minutes/60) + ($seconds/3600);
# Precision Six Integer to Decimal
my $decimal = $integer * .000001;
If you want to convert from decimal radians to degrees you can use
Math::Trig's rad2deg function.
METHODS
new
my $geo = new Geo::Distance;
my $geo = new Geo::Distance( no_units=>1 );
Returns a blessed Geo::Distance object. The new constructor accepts one
optional argument.
no_units - Whether or not to load the default units. Defaults to 0 (false).
kilometer, kilometre, meter, metre, centimeter, centimetre, millimeter,
millimetre, yard, foot, inch, light second, mile, nautical mile,
poppy seed, barleycorn, rod, pole, perch, chain, furlong, league,
fathom
formula
if($geo->formula eq 'hsin'){ ... }
$geo->formula('cos');
Allows you to retrieve and set the formula that is currently being used
to calculate distances. The availabel formulas are hsin, polar, cos, and
mt. hsin is the default and mt/cos are depreciated in favor of hsin.
polar should be used when calculating coordinates near the poles.
reg_unit
$geo->reg_unit( $radius, $key );
$geo->reg_unit( $key1 => $key2 );
$geo->reg_unit( $count1, $key1 => $key2 );
$geo->reg_unit( $key1 => $count2, $key2 );
$geo->reg_unit( $count1, $key1 => $count2, $key2 );
This method is used to create custom unit types. There are several ways
of calling it, depending on if you are defining the unit from scratch,
or if you are basing it off of an existing unit (such as saying 12
inches = 1 foot ). When defining a unit from scratch you pass the name
and rho (radius of the earth in that unit) value.
So, if you wanted to do your calculations in human adult steps you would
have to have an average human adult walk from the crust of the earth to
the core (ignore the fact that this is impossible). So, assuming we did
this and we came up with 43,200 steps, you'd do something like the
following.
# Define adult step unit.
$geo->reg_unit( 43200, 'adult step' );
# This can be read as "It takes 43,200 adult_steps to walk the radius of the earth".
Now, if you also wanted to do distances in baby steps you might think
"well, now I gotta get a baby to walk to the center of the earth". But,
you don't have to! If you do some research you'll find (no research was
actually conducted) that there are, on average, 4.7 baby steps in each
adult step.
# Define baby step unit.
$geo->reg_unit( 4.7, 'baby step' => 'adult step' );
# This can be read as "4.7 baby steps is the same as one adult step".
And if we were doing this in reverse and already had the baby step unit
but not the adult step, you would still use the exact same syntax as
above.
distance
my $distance = $geo->distance( 'unit_type', $lon1,$lat1 => $lon2,$lat2 );
Calculates the distance between two lon/lat points.
closest
my $locations = $geo->closest(
dbh => $dbh,
table => $table,
lon => $lon,
lat => $lat,
unit => $unit_type,
distance => $dist_in_unit
);
This method finds the closest locations within a certain distance and
returns an array reference with a hash for each location matched.
The closest method requires the following arguments:
dbh - a DBI database handle
table - a table within dbh that contains the locations to search
lon - the longitude of the center point
lat - the latitude of the center point
unit - the unit of measurement to use, such as "meter"
distance - the distance, in units, from the center point to find locations
The following arguments are optional:
lon_field - the name of the field in the table that contains the longitude, defaults to "lon"
lat_field - the name of the field in the table that contains the latitude, defaults to "lat"
fields - an array reference of extra field names that you would like returned with each location
where - additional rules for the where clause of the sql
bind - an array reference of bind variables to go with the placeholders in where
sort - whether to sort the locations by their distance, making the closest location the first returned
count - return at most these number of locations (implies sort => 1)
This method uses some very simplistic calculations to SQL select out of
the dbh. This means that the SQL should work fine on almost any database
(only tested on MySQL and SQLite so far) and this also means that it is
fast. Once this sub set of locations has been retrieved then more
precise calculations are made to narrow down the result set. Remember,
though, that the farther out your distance is, and the more locations in
the table, the slower your searches will be.
FORMULAS
Currently Geo::Distance only has spherical and flat type formulas. If
you have any information concerning ellipsoid and geoid formulas, the
author would much appreciate some links to this information.
tv: Thaddeus Vincenty Formula
This is a highly accurate ellipsoid formula. For most applications hsin
will be faster and accurate enough. I've read that this formula can be
accurate to within a few millimeters.
This formula is still considered alpha quality. It has not been tested
enough to be used in production.
hsin: Haversine Formula
dlon = lon2 - lon1
dlat = lat2 - lat1
a = (sin(dlat/2))^2 + cos(lat1) * cos(lat2) * (sin(dlon/2))^2
c = 2 * atan2( sqrt(a), sqrt(1-a) )
d = R * c
The hsin formula is the new standard formula for Geo::Distance because
of it's improved accuracy over the cos formula.
polar: Polar Coordinate Flat-Earth Formula
a = pi/2 - lat1
b = pi/2 - lat2
c = sqrt( a^2 + b^2 - 2 * a * b * cos(lon2 - lon1) )
d = R * c
While implimented, this formula has not been tested much. If you use it
PLEASE share your results with the author!
cos: Law of Cosines for Spherical Trigonometry
a = sin(lat1) * sin(lat2)
b = cos(lat1) * cos(lat2) * cos(lon2 - lon1)
c = arccos(a + b)
d = R * c
Although this formula is mathematically exact, it is unreliable for
small distances because the inverse cosine is ill-conditioned.
gcd: Great Circle Distance.
c = 2 * asin( sqrt(
( sin(( lat1 - lat2 )/2) )^2 +
cos( lat1 ) * cos( lat2 ) *
( sin(( lon1 - lon2 )/2) )^2
) )
Similar notes to the mt and cos formula, not too terribly accurate.
mt: Math::Trig great_circle_distance
This formula uses Meth::Trig's great_circle_distance function which at
this time uses math almost exactly the same as the cos formula. If you
want to use the cos formula you may find that mt will calculate faster
(untested assumption). For some reason mt and cos return slight
differences at very close distances. The mt formula has the same
drawbacks as the cos formula.
This is the same formula that was previously the only one used by
Geo::Distance (ending at version 0.06) and was wrongly called the "gcd"
formula.
Math::Trig states that the formula that it uses is:
lat0 = 90 degrees - phi0
lat1 = 90 degrees - phi1
d = R * arccos(cos(lat0) * cos(lat1) * cos(lon1 - lon01) + sin(lat0) * sin(lat1))
NOTES
If Geo::Distance::XS is installed, this module will use it. You can
stick with the pure Perl version by setting the GEO_DISTANCE_PP
environment variable before using this module.
TODO
* A second pass should be done in closest before distance calculations
are made that does an inner radius simplistic calculation to find
the locations that are obviously within the distance needed.
* Tests! We need more tests!
* For NASA-quality accuracy a geoid forumula.
* The closest() method needs to be more flexible and (among other
things) allow table joins.
SEE ALSO
Math::Trig - Inverse and hyperbolic trigonemetric Functions.
<http://www.census.gov/cgi-bin/geo/gisfaq?Q5.1> - A overview of
calculating distances.
<http://williams.best.vwh.net/avform.htm> - Aviation Formulary.
AUTHOR
Aran Clary Deltac <bluefeet@cpan.org>
CONTRIBUTORS
gray, <gray at cpan.org>
LICENSE
This library is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.