module provides two different approaches to
calculating great circle distances on the surface of the Earth. The one
described first depends on the
The second one is based on the built-in
using longitude and latitude for the coordinates.
In this module, the Earth is assumed to be perfectly spherical. (If that's too inaccurate for you, you might want to look at the PostGIS project.)
module must be installed
can be installed
(although you can use the
to install both in one command).
It is strongly recommended that
be installed in the same schema, and that
that schema be one for which CREATE privilege has not been and will not
be granted to any untrusted users.
Otherwise there are installation-time security hazards
's schema contains objects defined
by a hostile user.
Furthermore, when using
after installation, the entire search path should contain only trusted
F.13.1. Cube-Based Earth Distances
Data is stored in cubes that are points (both corners are the same) using 3
coordinates representing the x, y, and z distance from the center of the
Earth. A domain
is provided, which
includes constraint checks that the value meets these restrictions and
is reasonably close to the actual surface of the Earth.
The radius of the Earth is obtained from the
function. It is given in meters. But by changing this one function you can
change the module to use some other units, or to use a different value of
the radius that you feel is more appropriate.
This package has applications to astronomical databases as well.
Astronomers will probably want to change
to return a
so that distances are in degrees.
Functions are provided to support input in latitude and longitude (in degrees), to support output of latitude and longitude, to calculate the great circle distance between two points and to easily specify a bounding box usable for index searches.
The provided functions are shown in Table F.5 .
Table F.5. Cube-Based Earthdistance Functions
Returns the assumed radius of the Earth.
Converts the normal straight line (secant) distance between two points on the surface of the Earth to the great circle distance between them.
Converts the great circle distance between two points on the surface of the Earth to the normal straight line (secant) distance between them.
Returns the location of a point on the surface of the Earth given its latitude (argument 1) and longitude (argument 2) in degrees.
Returns the latitude in degrees of a point on the surface of the Earth.
Returns the longitude in degrees of a point on the surface of the Earth.
Returns the great circle distance between two points on the surface of the Earth.
Returns a box suitable for an indexed search using the
F.13.2. Point-Based Earth Distances
The second part of the module relies on representing Earth locations as
values of type
, in which the first component is taken to
represent longitude in degrees, and the second component is taken to
represent latitude in degrees. Points are taken as (longitude, latitude)
and not vice versa because longitude is closer to the intuitive idea of
x-axis and latitude to y-axis.
A single operator is provided, shown in Table F.6 .
Table F.6. Point-Based Earthdistance Operators
Computes the distance in statute miles between two points on the Earth's surface.
Note that unlike the
-based part of the module, units
are hardwired here: changing the
not affect the results of this operator.
One disadvantage of the longitude/latitude representation is that
you need to be careful about the edge conditions near the poles
and near +/- 180 degrees of longitude. The
representation avoids these discontinuities.