This contrib package contains two different approaches to calculating
great circle distances on the surface of the Earth. The one described
first depends on the contrib/cube package (which MUST be installed before
earthdistance is installed). The second one is based on the point
datatype using latitude and longitude for the coordinates. The install
script makes the defined functions executable by anyone.

Make sure contrib/cube has been installed.
make
make install
make installcheck

To use these functions in a particular database as a postgres superuser do:
psql databasename < earthdistance.sql

-------------------------------------------
contrib/cube based Earth distance functions
Bruno Wolff III
September 2002

A spherical model of the Earth is used.

Data is stored in cubes that are points (both corners are the same) using 3
coordinates representing the distance from the center 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 it
to use some other units or to use a different value of the radius
that you feel is more appropiate.

This package also 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 allow for input in latitude and longitude (in
degrees), to allow for 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 functions are all 'sql' functions. If you want to make these functions
executable by other people you will also have to make the referenced
cube functions executable. cube(text), cube_distance(cube,cube),
cube_ll_coord(cube,int) and cube_enlarge(cube,float8,int) are used indirectly
by the earth distance functions. is_point(cube) and cube_dim(cube) are used
in suggested constraints for data in domain earth. cube_ur_coord(cube,int)
is used in the regression tests and might be useful for looking at bounding
box coordinates in user applications.

A domain of type cube named earth is defined. Since check constraints
are not supported for domains yet, this isn't as useful as it might be.
However the checks that should be supplied to all data of type earth are:

constraint not_point check(is_point(earth))
constraint not_3d check(cube_dim(earth) <= 3)
constraint on_surface check(abs(cube_distance(earth, '(0)'::cube) /
           earth() - 1) < '10e-12'::float8);

The following functions are provided:

earth() - Returns the radius of the earth in meters.

sec_to_gc(float8) - Converts the normal straight line (secant) distance between
between two points on the surface of the Earth to the great circle distance
between them.

gc_to_sec(float8) - Converts the great circle distance between two points
on the surface of the Earth to the normal straight line (secant) distance
between them.

ll_to_cube(float8, float8) - Returns the location of a point on the surface of
the Earth given its latitude (argument 1) and longitude (argument 2) in degrees.

latitude(earth) - Returns the latitude in degrees of a point on the surface
of the Earth.

longitude(earth) - Returns the longitude in degrees of a point on the surface
of the Earth.

earth_distance(earth, earth) - Returns the great circle distance between
two points on the surface of the Earth.

earth_box(earth, float8)  - Returns a box suitable for an indexed search using
the cube @ operator for points within a given great circle distance of a
location. Some points in this box are further than the specified great circle
distance from the location so a second check using earth_distance should be
made at the same time.

One advantage of using cube representation over a point using latitude and
longitude for coordinates, is that you don't have to worry about special
conditions at +/- 180 degrees of longitude or near the poles.

Below is the documentation for the Earth distance operator that works
with the point data type.

---------------------------------------------------------------------

I corrected a bug in the geo_distance code where two double constants
were declared as int. I also changed the distance function to use
the haversine formula which is more accurate for small distances.
Bruno Wolff
September 2002

---------------------------------------------------------------------

Date: Wed, 1 Apr 1998 15:19:32 -0600 (CST)
From: Hal Snyder <hal@vailsys.com>
To: vmehr@ctp.com
Subject: [QUESTIONS] Re: Spatial data, R-Trees

> From: Vivek Mehra <vmehr@ctp.com>
> Date: Wed, 1 Apr 1998 10:06:50 -0500

>  Am just starting out with PostgreSQL and would like to learn more about
> the spatial data handling ablilities of postgreSQL - in terms of using
> R-tree indexes, user defined types, operators and functions. 
> 
> Would you be able to suggest where I could find some code and SQL to
> look at to create these?

Here's the setup for adding an operator '<@>' to give distance in
statute miles between two points on the earth's surface. Coordinates
are in degrees. Points are taken as (longitude, latitude) and not vice
versa as longitude is closer to the intuitive idea of x-axis and
latitude to y-axis.

There's C source, Makefile for FreeBSD, and SQL for installing and
testing the function.

Let me know if anything looks fishy!

A note on testing C extensions - it seems not enough to drop a function
and re-create it - if I change a function, I have to stop and restart
the backend for the new version to be seen. I guess it would be too
messy to track which functions are added from a .so and do a dlclose
when the last one is dropped.
