[postgis-users] How to locate adjacent polygons?

[Colin]

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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