Basic GIS calculations that may be useful in Sahana eden:

* Centroid
select name, asText(Centroid(the_geom)) from table where name='location';

* Bounding boxes
select name, asText(Envelope(the_geom) from table where name='location';
select name, extent(the_geom) from table where name = 'location' group by name;

* buffer calculations

* Distance measurements
select distance(
(transform(GeomFromText('Point(lon lat)'), 900913), 26915), 
(transform(GeomFromText('Point(lon lat)'), 900913), 26915);

* intersections
select name from location where
GeomFromText('Point(lon lat)',900913)
&& the_geom AND
within (GeomFromText('Point(lon lat)', 900913), the_geom);

* Point within polygon
select name from location where
GeomFromText('Point(lon lat)', 900913)
&& the_geom;

* Spherical distance
select distance_sphere(
(transform(GeomFromText('Point(lon lat)'), 900913), 
(transform(GeomFromText('Point(lon lat)'), 900913);

* Spheroid distance
select distance_spheroid(
(GeomFromText('Point(lon lat)'), 900913), 
(GeomFromText('Point(lon lat)'), 900913),
'SPHEROID["GRS 1980", 6378137, 298.257222101]');