[postgis-devel] Re: [postgis-users] ST_MakeBox2D losing precision ?

[Kevin Neufeld]

We should probably clarify the docs for these types of methods involving 
BOX2D.

-------------
Kevin Neufeld
Software Developer
Refractions Research Inc.
300-1207 Douglas St.
Victoria, B.C., V8W 2E7

Phone: (250) 383-3022
Email: kneufeld at refractions.net



Mark Cave-Ayland wrote:
> On Fri, 2007-10-12 at 12:56 +0200,
> postgis.domain.thewild at spamgourmet.com wrote:
>   
>> Hi list !
>>
>> First of all, the versions :
>> PostgreSQL : "PostgreSQL 8.2.0 on i686-pc-mingw32, compiled by GCC gcc.exe (GCC) 3.4.2 (mingw-special)"
>> PostGIS : "POSTGIS="1.3.1" GEOS="3.0.0rc4-CAPI-1.3.3" PROJ="Rel. 4.5.0, 22 Oct 2006" USE_STATS"
>>
>> I am trying to modify geometries stored in a table.
>> They all should be squares with 158.75 meters sides, but some are bigger (vertically). I wrote a query to update them, but I realized that after the update, some geometries where 159 meters high.
>>
>> Here is the test that shows this behaviour :
>>
>> SELECT 
>> 	ymax(makebox2d(st_makepoint(xmin(bbox),ymax(bbox)-158.75), st_makepoint(xmax(bbox),ymax(bbox)))) - 
>> 	ymin(makebox2d(st_makepoint(xmin(bbox),ymax(bbox)-158.75), st_makepoint(xmax(bbox),ymax(bbox))))
>> FROM 
>> 	my_table
>> WHERE 
>> 	(st_ymax(bbox)-st_ymin(bbox)) > 158.75;
>>
>> Results are 158.5 or 159, so it appears that ST_MakeBox2D is losing part of the precision of my points.
>> I checked the points coordinates returned by ST_MakePoint, and those are OK !
>>
>> Have I overlooked something ?
>> I will rewrite this to create a polygon WKT, but I wanted to point this out.
>>
>>
>> Regards
>> --
>> Arnaud
>>     
>
>
> Hi Arnaud,
>
> The BOX2D datatype is "lossy" since it is intended only to be used
> within the GiST indices where doubles are "rounded up" to the nearest
> float. Probably the easiest thing to try is working with native
> geometries rather than the BOXXD types - you may find that
> ST_Translate() helps for what you are trying to do.
>
>
> HTH,
>
> Mark.
>
>