[postgis-users] How to locate adjacent polygons?

[Nicklas Avén]

Hallo Colin

Is it you that has this question at gis.stackexchange.com?
http://gis.stackexchange.com/questions/6098/how-do-you-produce-a-output-table-containing-distances-between-mineral-grains-at

Or at least I saw some similarity in the questions. 

>From what you write here, if I understand you right?

If you want to test polygonA, you want every polygon that gets some
light from polygonA if it is PlygonA is shining. 

That might be quite complicated. 

/Nicklas





On Wed, 2011-02-16 at 13:09 +0000, Colin wrote:
> 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
> 
>