[postgis-users] polygons to linestrings

[Rémi Cura]

Hey,
you have to keep the path of the dumped segments, and carefully choose
which duplicates to keep, inorder to be able to reconstruct in the right
order.

(for the rest I'm referring to this function :
https://github.com/Remi-C/PPPP_utilities/blob/master/postgis/rc_DumpSegments.sql
)

Supposing that your polygon have an unique ID, and that you only consider
there boundaries (interior ring couldn't be shared as they can't touch the
boundaries and you said you have no overlaps)

_convert polygon to boundaries, generating a unique ID per
polygon/boundaries
_break each boundaries into segment and keep the path of each segment (for
instance with my function)
_now you can remove duplicate, but you have to order the querry so has to
keep continuous parts
_Then you can recompose your geometry :

Here is an example :
---------

 WITH the_geom AS ( --creating a fake geom for test purpose, function
available here :
https://github.com/Remi-C/PPPP_utilities/blob/master/postgis/cdb_GenerateGrid.sql
        SELECT  row_number() over() AS id, geom
        FROM CDB_RectangleGrid(ST_GeomFromtext('polygon((0 0, 100 0, 100
100, 0 100 , 0 0))'), 10,10) AS geom
 )
 ,dmp_seg AS ( --breaking the boundary of polygons into segments. Function
is available here:
https://github.com/Remi-C/PPPP_utilities/blob/master/postgis/rc_DumpSegments.sql
     SELECT id, rc_DumpSegments(ST_Boundary(geom)) as dmpgeom
     FROm the_geom
 )
,cleaned_ds AS ( --snapping to grid to avoid precision issue, replace 0.1
by your alloxed precision
 SELECT id, (dmpgeom).path, ST_SNapToGrid((dmpgeom).geom,0.1) as geom
 FROM dmp_seg
 )
,dedup AS ( --deleting the duplicates in the segments, but not randomly :
provide an order to be able ot reconstruct after
 SELECT DISTINCT ON ( geom  )  *
 FROM cleaned_ds
 ORDER BY  geom, id, path
 ) --reconstructing lines from segment, but again with the right order
 ,reconstructed_lines AS (
 SELECT id,   ST_MakeLine(array_agg(geom ORDER BY  dedup.path) ) as geom
 FROM dedup
GROUP BY id
 ) --simple check, can be suppressed : it should output no row
 SELECT id, geom
 FROM reconstructed_lines
 WHERE st_IsValid(geom)  = FALSE
---------

Cheers,

Rémi-C



2014-05-23 6:05 GMT+02:00 Roxanne Reid-Bennett <rox at tara-lu.com>:

>  On 5/22/2014 12:28 PM, Richard Greenwood wrote:
>
>  I have a polygon table that I want to convert to linestrings without any
> duplicated (overlapping) linestrings. I'm doing this simply to improve
> cartography so I'm looking for a quick and dirty approach, avoiding
> topology if possible.
>
>
> I'm not sure I understand what it is you are attempting to do from this
> description.  I would expect lines going in two directions from each
> polygon and I just don't know how you would use that to help with "improve
> cartography".
>
>
>  The closest I've gotten so far is to break the polygons boundaries into
> simple (two vertex) lines and delete all the duplicate lines. Now I'm stuck
> getting the simple lines back into polylines. Just doing a union of the
> whole mess isn't getting me individual linestrings.
>
> So, crazy "out of the blue" question.. have  you considered creating a
> line (or lines) from the centroids?
>
>
>  All this is turning into more steps that I anticipated and I wonder if
> I'm missing simpler approach?
>
> Or maybe describe what your target is?  There are a lot of people a lot
> more knowledgeable than I on this list, and if they haven't answered,
> perhaps you haven't presented your challenge in a manner that they can
> address.
>
> Roxanne
>
>
>  Thanks,
> Rich
>
> --
> Richard W. Greenwood, PLS
> www.greenwoodmap.com
>
>
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