F.15. earthdistance
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).
Caution
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.
F.15.1. 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
earth
over type
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
F.15.2. 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
|
Operator Description |
|---|
|
Computes 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.