[postgis-users] Units for distance function
mark.cave-ayland at ilande.co.uk
Thu Jul 26 22:26:36 PDT 2007
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
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
ILande - Open Source Consultancy
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