[postgis-devel] Re: [postgis-users] Box2D on Postgis 1.0.0 RC1

strk at refractions.net strk at refractions.net
Mon Jan 31 01:23:18 PST 2005


I've changed BOX2D output function to use default float precision
(6 significant digits). 

--strk;

On Mon, Jan 31, 2005 at 08:14:19AM +0100, Hubert Fröhlich wrote:
> 
> Hi strk,
> 
> 
> >It is untouched in RC2, and it's not a bug.
> >
> >BOX2D use float values, BOX3D uses doubles.
> >An algorithm is used to ensure a BOX2D derived from a BOX3D
> >always covers an area at least as big. 15 decimal digits
> >output is forced on BOX2D output. 
> >
> >Check these:
> >
> >strk=# select box2d('BOX(0.4 -0.03,12.03 12.03)');
> >                                    box2d
> >------------------------------------------------------------------------------
> > BOX(0.400000005960464 -0.0299999993294477,12.0299997329712 
> > 12.0299997329712)
> >(1 row)
> >
> >As you can see your numbers, expressed in decimal form, when converted
> >to binary loose that 'roundness', in this case they become higher
> >for the "min" values and lower for the "max" values, practically resulting
> >in a smaller area.
> >
> >
> >strk=# select box2d('BOX3D(0.4 -0.03 0,12.03 12.03 0)'::box3d);
> >                                    box2d
> >------------------------------------------------------------------------------
> > BOX(0.399999976158142 -0.0300000011920929,12.0300006866455 
> > 12.0300006866455)
> >(1 row)
> >
> >Here the algorithm ensured the BOX2D actually covers at least the area
> >covered by the input BOX3D.
> >
> >If we didn't force 15 decimal digits users would not notice this,
> >and maybe that would be a good thing. How many users care about precision
> >of the text form for BOX3D and BOX2D ?
> >
> 
> Thanks for the clarification. Maybe you could add some sentence to the 
> docs?
> 
> Greetings, Hubert
> -- 
> -------------------------------------------------------------------------------
> Dr.-Ing. Hubert Fröhlich			
> Bezirksfinanzdirektion München 			
> Alexandrastr. 3, D-80538 München, GERMANY
> Tel. :+49 (0)89 / 2190 - 2980
> Fax  :+49 (0)89 / 2190 - 2997
> hubert dot froehlich at bvv dot bayern dot de
> 
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