[postgis-tickets] [PostGIS] #4856: ST_Intersection error

[PostGIS]

#4856: ST_Intersection error
-----------------------+---------------------------
  Reporter:  ezimanyi  |      Owner:  pramsey
      Type:  defect    |     Status:  new
  Priority:  medium    |  Milestone:  PostGIS 2.5.6
 Component:  postgis   |    Version:  2.5.x
Resolution:            |   Keywords:
-----------------------+---------------------------

Comment (by mdavis):

 This is not a bug, it is actually expected behaviour.  The reason is
 because the floating point representation used internally for numbers is
 not able to precisely represent certain decimal numbers (even apparently
 simple ones like 2.6).  The usual term for this is "roundoff error".

 Simplifying the example makes this more clear:
 {{{
 SELECT ST_AsTExt(ST_Intersection('LINESTRING(1 1,3 3.5)','LINESTRING(1
 1,2.6 3)'));
 -- POINT(1 1)
 }}}
 This is actually the correct result, since the point (2.6 3) cannot be
 represented exactly internally, and so it does not lie exactly on the
 first line. This can be seen from:
 {{{
 SELECT ST_Intersects('LINESTRING (1 1, 3 3.5)', 'POINT (2.6 3)');
 -- f
 }}}
 In this case the `intersection` and `intersects` operations are
 consistent. They just don't match what you think they should be. But you
 are computing in exact decimal arithmetic!

 One way to mitigate this issue is to use the new ST_Intersection function
 that takes a gridsize. Using a small gridsize will cause this operation to
 return the "expected" result, since it causes the point to snap to the
 line.

 Incidentally, there are also situations where the operations themeselves
 are internally inconsistent, also due to roundoff error.  This is a
 classic example:
 {{{
 WITH a AS (
   SELECT 'LINESTRING(0.2 0.2, 1.2 1.2)'::geometry AS l1,
          'LINESTRING(1.2 0.2, 0.2 1.2)'::geometry AS l2
 )
 SELECT ST_AsText(ST_Intersection(l1, l2)) AS intersection_l1_l2,
        ST_Intersects(ST_Intersection(l1, l2), l1) AS
 l1_intersects_intersection,
        ST_Intersects(ST_Intersection(l1, l2), l2) AS
 l2_intersects_intersection
 FROM a;
 ---
 -[ RECORD 1 ]--------------+---------------
 intersection_l1_l2         | POINT(0.7 0.7)
 l1_intersects_intersection | t
 l2_intersects_intersection | f
 }}}

-- 
Ticket URL: <https://trac.osgeo.org/postgis/ticket/4856#comment:1>
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