[postgis-users] meaning of distance for unprojected data?

[strk at refractions.net]

On Mon, Sep 19, 2005 at 01:38:59PM +1000, Duana Stanley wrote:
> When I have unprojected data (SRID = -1) I can still perform the spatial
> query distance() on two geoms even though the documentation for the
> distance() function says cartesian distance using projected units.
> 
> Why does it not produce an error when I ask for the distance between two
> geoms with SRID = -1?  Is there a default GCS used when data is
> unprojected?

The distance function has no knowledge about projections, and
doesn't care about units semantic. It uses cartesian math,
correctness of result will depend on the *real* projection of
the data. If the projection is isometric distance would be closer
to reality.

--strk;