[postgis-users] How to locate adjacent polygons?

[Colin]

thanks all for replies
my apologies for confused replies and stringing the thread out

Nick
No that wasnt me at stackexchange. altho the graphic shows a
similar scenario, except mine are a little bit more evenly sized and
distributed.

Brett
yea thats pretty much where I'm at... except I hadnt considered
dropping to foreach vertex.
Altho even then I can see that there might well be cases where count
would be >2 for all vertexes in 'adjacent' polygons.

Steven
Re your diagram: Only A would be considered a neighbour of E
And I agree only a raytracing type solution will give a complete solution. But its not justified.

I think extending my current solution with Brents suggestion of foreach(vertex) is the best compromise.
I feel there are sufficient vertices to identify a high percentage of neighbours.

And since 99% is prob OK for this project, I guess thats the way to go.

thanks all

--
Colin


On Wed, 16 Feb 2011 10:13 -0800, pcreso at pcreso.com wrote:

  Hi Colin,
  Given your definition of adjacent, you are probably better off
  with a script using sqls to retrieve data/write result & the
  script to implement the logic, iterating through the polygons:

  foreach polygon (a)
     foreach different polygon (b)
        foreach a.vertex
           foreach b.vertex
             get count of polygons intersected by a line between
  a.vertex and b.vertex
             if count > 2 then (a) and (b) are not adjacent
         next b.vertex
      next (a) vertex
      write result
    next (b) polygon
  next (a) polygon
  This would define adjacency as having no intervening polygons
  between any two polygons, partial adjacency (where parts of two
  polygons are & other parts are separated) would require a slight
  change in the logic, so if any a.vertex & b.vertex have no
  intervening polygons, then (a) and (b) are adjacent. How you
  define & implement adjacency is up to you. As is the choice of
  scripting language :-)
  HTH,
     Brent Wood

    Subject: Re: [postgis-users] How to locate adjacent polygons?
    To: postgis-users at postgis.refractions.net
    Date: Thursday, February 17, 2011, 2:09 AM

  sorry for double post - working from dodgy pda
  Hi Steve / Brent
  Thanks for replies
  Steve: Yes I did look at buffer but I discounted it because a
  small
  number of the polys are at distances well outside the mean and I
  didnt
  see any easy way of including them.
  Brents solution with st_distance and a tolerance factor might
  suffer
  from the same problem
  And in fact the definition of adjacent doesnt include distance -
  its
  simply that no other polygon crosses some path between any 2
  I also have the centroids calced and stored so in my spatial
  innocence i
  devised a solution which almost works and uses
  ST_Crosses(ST_MakeLine
  something like this:
  select a.id, b.id, ST_Crosses(ST_MakeLine(a.centroid,
  b.centroid),
  b.geom) as stcrosses
  from a, b where a.id = nnn and a.id!=b.id order by stcrosses
  I run this per id from program code. I then filter the return to
  give
  adjacents
  I'm not at my dev machine right now and I suspect thats not
  exacty
  right(at all!) but what I have isnt affected by distance and
  almost
  works - except occasionally for the very complex and where there
  are
  larger gaps between polys.
  so maybe a combination of these methods?
  Or since the data wont change often, maybe I just plot them in
  qgis and
  do it manually! ;-)
  Colin
  On 02/16/2011 05:12 AM, Stephen Woodbridge wrote:
  > Colin,
  >
  > Did you look at buffer?
  >
  > Not tested but something like this might work where b.id are
  the
  > adjacent id's to a.id with the distance of <tolerance>.
  >
  > select a.id, b.id from mypolys a, mypolys b
  > where a.id != b.id
  > and buffer(a.geom, <tolerance>) && b.geom
  > and intersects(buffer(a.geom, <tolerance>), b.geom)
  >
  > -Steve
  >
  > On 2/15/2011 5:38 PM, Colin wrote:
  >> Hi
  >>
  >> I'm quite new to postgis and spatial databases.
  >> competent with sql and db's
  >>
  >> My problem: How to locate adjacent polygons.
  >>
  >> I have around around 2k irregular polygons.
  >> They've have been calculated as alpha / concave hulls from
  point sets.
  >> They're saved into pg as multipolygons
  >>
  >> id | description | geom
  >>
  >> The polygons dont have any regularity with regard to location
  and
  >> interaction
  >> the majority are close to a neighbour, but not touching.
  Typically
  >> within +/- 1 - 5% of polygon max dim
  >> a small number slightly overlap 1 or more neighbours, usually
  to quite a
  >> small extent
  >> their irregular shape can include 'undercuts'
  >>
  >>
  >> I need to identify the adjacent neighbours for each polygon
  >>
  >> I looked at various methods that might allow me to do this but
  I cant
  >> get a 100% solution
  >>
  >> Can anyone suggest methods that might work
  >>
  >> thanks
  >>
  >>
  >> Colin
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  Colin
  colinflack at fastmail.co.uk

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