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package GIS::Distance::Cosine;
use 5.008001;
use strictures 2;
our $VERSION = '0.20';
use parent 'GIS::Distance::Formula';
use Math::Trig qw( deg2rad acos );
use GIS::Distance::Constants qw( :all );
use namespace::clean;
sub _distance {
my ($lat1, $lon1, $lat2, $lon2) = @_;
$lon1 = deg2rad($lon1);
$lat1 = deg2rad($lat1);
$lon2 = deg2rad($lon2);
$lat2 = deg2rad($lat2);
my $a = sin($lat1) * sin($lat2);
my $b = cos($lat1) * cos($lat2) * cos($lon2 - $lon1);
my $c = acos($a + $b);
return $KILOMETER_RHO * $c;
}
1;
__END__
=encoding utf8
=head1 NAME
GIS::Distance::Cosine - Spherical law of cosines distance calculations.
=head1 DESCRIPTION
Although this formula is mathematically exact, it is unreliable for
small distances. See L<GIS::Distance::MathTrig> for related details.
A faster (XS) version of this formula is available as
L<GIS::Distance::Fast::Cosine>.
Normally this module is not used directly. Instead L<GIS::Distance>
is used which in turn interfaces with the various formula classes.
=head1 FORMULA
a = sin(lat1) * sin(lat2)
b = cos(lat1) * cos(lat2) * cos(lon2 - lon1)
c = arccos(a + b)
d = R * c
=head1 SEE ALSO
=over
=item *
L<https://en.wikipedia.org/wiki/Spherical_law_of_cosines>
=back
=head1 SUPPORT
See L<GIS::Distance/SUPPORT>.
=head1 AUTHORS
See L<GIS::Distance/AUTHORS>.
=head1 LICENSE
See L<GIS::Distance/LICENSE>.
=cut
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