[OSM-dev] osm2pgsql / postgis precision

[Jan-Benedict Glaw]

On Wed, 2009-07-01 19:13:26 +0100, Jon Burgess <jburgess777 at googlemail.com> wrote:
> On Wed, 2009-07-01 at 13:51 +0200, Etienne Chové wrote:
> > Etienne Chové a écrit :
> > > The difference is not really significant but postgis says
> > > polygons are olvelapping each other while they have just a
> > > common boundary.
> > 
> > The errors doesn't appear when I delete lines "#define
> > FIXED_POINT" in middle_ram.c and middle_pgsql.c
> > 
> > Is it danerous to delete them ?
> 
> I have come across this issue before, the 'slim' mode can fetch the node
> positions from the DB or from a RAM cache. The DB stores the positions
> in doubles (giving 53 bits of precision), the RAM cache uses fixed point
> arithmetic giving around 32 bits of precision. 32 bits is perfectly
> sufficient for OSM and halves the storage size.

Are you 100% sure about that comment?

ISTR that at some time, I calculated the accuracy of 32bit (float) at
the aequator, which resulted in +/- 15cm or the like. This may or may
not be enough...

MfG, JBG

-- 
      Jan-Benedict Glaw      jbglaw at lug-owl.de              +49-172-7608481
 Signature of:                    Arroganz verkürzt fruchtlose Gespräche.
 the second  :                                   -- Jan-Benedict Glaw
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