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 (which
must be installed before
earthdistance can be
installed). 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.)
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
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
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
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
F.13.2. Point-based Earth Distances
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
|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
representation avoids these discontinuities.