[postgis-users] WKT/WKB Calculations

Paragon Corporation lr at pcorp.us
Mon Sep 28 01:01:59 PDT 2009


The WKT truncates I think at about the 15th digit where as the wkb doesn't.
So yes you will lose information if you convert to WKT and have a lot of
decimal places.

Leo 

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Michael
Orlitzky
Sent: Sunday, September 27, 2009 10:39 PM
To: postgis-users at postgis.refractions.net
Subject: [postgis-users] WKT/WKB Calculations

I noticed that some error is introduced when converting from WKB to WKT (and
back). For example, the following computation of a polygon's area, divided
by itself, should return one:

   SELECT (ST_Area(ST_GeomFromEWKB(ST_AsEWKB(tiger.the_geom)))
          / ST_Area(tiger.the_geom))
          AS area_ratio
   FROM tiger

       area_ratio
   -------------------
                    1
                    1
                    1
                    .
                    .


But when I convert to WKT as an intermediate format, the results get thrown
off by as much as 0.1% (I realize the first couple of rows don't show this,
but my tests break when the error passes that threshold):

   SELECT (ST_Area(ST_GeomFromEWKT(ST_AsEWKT(tiger.the_geom)))
          / ST_Area(tiger.the_geom))
          AS area_ratio
   FROM tiger;

       area_ratio
   -------------------
                    1
     1.00000014105231
     1.00000017733961
     1.00000021839733
     1.00000004515224
	        etc.


Is this just a floating point limitation, or something else entirely?
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