[postgis-devel] [PostGIS] #1502: Unrobustness in distance calculations

[PostGIS]

#1502: Unrobustness in distance calculations
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 Reporter:  nicklas  |       Owner:  nicklas      
     Type:  defect   |      Status:  new          
 Priority:  medium   |   Milestone:  PostGIS 2.0.0
Component:  postgis  |     Version:  1.5.X        
 Keywords:           |  
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 This one is quite ugly.



 {{{
 SELECT st_dwithin(a,b,0), st_intersects(a,b),st_distance(a,b)  from
 (select 'LINESTRING(-97364 -97364, 9736.4 9736.4)'::geometry a, 'POINT(0
 0)'::geometry b ) f;
 }}}
 returns :

 f;t;3.086927381169e-11

 This is also point-segment problem like #1459

 The problem is that the distance is calculated by projecting the point to
 the segment and then calculating the distance from the projected point to
 the original point.

 The projection of the point to the segment is done by calculating where
 along the segment from 0 to 1 that the point should be projected. That
 value, r, doesn't have enough precision when the segment is long and the
 distance to between the original point is small.

 This was not a problem pre 1.5 because then the projected point was never
 calculated.

-- 
Ticket URL: <http://trac.osgeo.org/postgis/ticket/1502>
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