On Fri, 2007-07-27 at 01:49 +0100, Dave Potts wrote: > What units does the distance function use? am I correct in thinking its > derrived from the srid type of the geometry column? > > So in the case of the British national grid srid 2770 the correct unit > to use is the meter > > Dave.
Nearly... OS data plays nicely with SRID 27700, and the grid is specified in metres. One word of warning: PROJ.4 versions before 4.5.0 had a bug in the PROJ.4 parameters (a missing OSGB datum) which gave a drift of approximately ~80m compared to other datasets - this one had me scratching my head for a couple of days. So in short: make sure that you get PROJ.4 >= 4.5.0 and make sure that you add the parameter "+datum=OSGB36" to your spatial_ref_sys proj4text field for SRID 27700 if it isn't there already - see http://lists.maptools.org/pipermail/proj/2006-May/002265.html for the gory details (the +datum=airy was replaced by +datum=OSGB36 in the final commit). HTH, Mark. -- ILande - Open Source Consultancy http://www.ilande.co.uk _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users
