[postgis-users] st_buffer (radius_of_buffer)

[Kevin Neufeld]

? Sorry, I'm confused.  I would think you would get a ERROR:  Operation on two geometries with different SRIDs when running your query.

You are taking a lat/long coordinate, transforming it to a [deprecated] mercator projection and storing the results in a table called route.

Then you are asking if the mercator point, when buffered by 0, intersects a point in lat/long.

If you meant to include a transform back to 4326 in the intersect query (ie. st_intersects(st_buffer(st_transform(route.the_geom,4326),0.0), ... ), you should be aware that transforming a geometry back and forth between projections will likely lead to rounding errors.

ie.
SELECT
   astext(g1) g1,
   astext(g2) g2,
   g1,
   g2,
   g1 ~= g2,
   st_intersects(g1, 'SRID=4326;POINT(30 20)'),
   st_intersects(g2, 'SRID=4326;POINT(30 20)')
FROM (
   SELECT geom g1, st_transform(st_transform(geom,900913),4326) g2
   FROM (
     SELECT 'SRID=4326;multilinestring((30 20,31 21,32 22))'::geometry AS geom
   ) a
) b;
-[ RECORD 1 ]-+---------------------------------------------------------------------------------------------------------------------------------------------
g1            | MULTILINESTRING((30 20,31 21,32 22))
g2            | MULTILINESTRING((30 20,31 21,32 22))
g1            | 0105000020E6100000010000000102000000030000000000000000003E4000000000000034400000000000003F40000000000000354000000000000040400000000000003640
g2            | 0105000020E610000001000000010200000003000000FEFFFFFFFFFF3D40FDFFFFFFFFFF33400200000000003F400100000000003540FEFFFFFFFFFF3F40FEFFFFFFFFFF3540
equals        | f
st_intersects | t
st_intersects | f


Notice how your geometry, when transform to 900913 and back to 4326 looks the same in text, but are in fact different in binary?  The transformed geometry, does not intersect (30,20).  It might if you buffer it by 0.1.

Cheers,
Kevin

ibrahim saricicek wrote:
> hi all;
> 
> I think 0.0 of radius means the object itself? Am i wrong?
> 
> I'm inserting a row to my route table
> 
> insert into route (the_geom) select 
> st_transform(ST_GeomFromText('SRID=4326;multilinestring((30 20,31 21,32 
> 22))'),900913)
> 
> why the result of this is false? that is a point on my route. what is 
> defined with '0.0 of radius'?
> the result is true for 0.1 to 0.000000001? Why for 0.0000000001 radius, 
> result is false?
> 
> select
> st_intersects(st_buffer(route.the_geom,0.0), st_geomFromText('POINT(30 
> 20)',4326)) 
> as result
> from route
> 
> 
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