[postgis-devel] [PostGIS] #231: New distance-calculations proposal

[PostGIS]

#231: New distance-calculations proposal
--------------------------+-------------------------------------------------
  Reporter:  nicklas      |       Owner:  robe         
      Type:  enhancement  |      Status:  assigned     
  Priority:  medium       |   Milestone:  postgis 1.5.0
 Component:  postgis      |     Version:               
Resolution:               |    Keywords:               
--------------------------+-------------------------------------------------
Comment (by nicklas):

 As I have mentioned, I have had an idea about better subgeometry-handling
 in the distance-calculations. Now I have come up with a first try.[[BR]]
 [[BR]]
 What it does, simplified is:[[BR]]
 1)  unwinds all subgeometries and puts them in a list without
 hiearchy.[[BR]]
 2)  calculates bboxes for the subgeometries[[BR]]
 3)  order geometries from distances between bboxes[[BR]]
 4)  calculates real distances in list order until distance between bboxes
 is bigger than smallest found distance between real geometries.
 [[BR]][[BR]]

 For single geometries this might give a little overhead, but it doesn't
 seem to be noticable. There are still som things to do to let the single
 geometries passing by.
 [[BR]][[BR]]

 The best thing about this I think is the possibility to collect a lot of
 geometries and fast get the distance or with shortestline get a line to
 the closest geometry.
 [[BR]]
 One examples to show why I belive in the idea.
 From the dataset Regina put in #137
 [[BR]]
 To find the closest point in the rest of USA from Alaska we can then do:
 [[BR]]
 {{{

 select st_distance(a.the_geom, b.the_geom),
   st_endpoint(st_shortestline(a.the_geom, b.the_geom)) as the_geom
     from
 (select st_collect(the_geom) as the_geom
   from us.states where state != 'Alaska') a,
 (select the_geom as the_geom
   from us.states where state = 'Alaska') b

 }}}

 Querytime at my laptop is less than 400ms and then we have compared alaska
 with 52 other states.
 [[BR]]


 The patch subgeomhandling applies to my spike
 [[BR]]

 It is probably not stable yet, but is it a good idea?
 [[BR]]
 /Nicklas

-- 
Ticket URL: <http://trac.osgeo.org/postgis/ticket/231#comment:36>
PostGIS <http://trac.osgeo.org/postgis/>
PostGIS