The earthdistance module provides two different approaches to
calculating great circle distances on the surface of the Earth. The one
described first depends on the cube module.
The second one is based on the built-in point data type,
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.)
The cube module must be installed
before earthdistance can be installed
(although you can use the CASCADE option
of CREATE EXTENSION to install both in one command).
It is strongly recommended that earthdistance
and cube 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
if earthdistance's schema contains objects defined
by a hostile user.
Furthermore, when using earthdistance's functions
after installation, the entire search path should contain only trusted
schemas.
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 earth over cube 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 earth()
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 earth() to return a
radius of 180/pi() 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
The second part of the module relies on representing Earth locations as
values of type point, 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
| Operator | Returns | Description |
|---|---|---|
point <@> point | float8 | Gives the distance in statute miles between two points on the Earth's surface. |
Note that unlike the cube-based part of the module, units
are hardwired here: changing the earth() function will
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 cube-based
representation avoids these discontinuities.