[postgis-users] unable to assign hole to a shell

[Martin Davis]

Yep, absolutely right - I meant Union, not intersection!

Kevin Neufeld wrote:
>
> Martin Davis wrote:
>>>  
>>>> I was able to rebuild your polygon this way:
>>>> 1. extract the exterior ring - you may want to add to this subquery 
>>>> all
>>>> the interior rings if you have any
>>>> 2. extract all the points of the ext ring as ordered pairs (ie as
>>>> point1-point2, point2-point3, point3-point4, ....)
>>>> 3. recreate line segments from all these ordered point pairs, removing
>>>> duplicates.
>>>> 4. rebuild the polygon using st_buildarea()
>>>>     
>>>
>>> Would this trick work on every polygon? By construction my polygons 
>>> have no interior ring.
>>>   
>> Knowing that you have no holes definitely helps.
>>
>> Another thing you might be able to try is to:
>> - convert the polygon to a linestring
>> - call intersection on it with a single point from the linestring 
>> (this effectively nodes the linework)
>> - polyognize the resulting linework back to a polygon  
> Nice. You mean union, not intersection though, right Martin? Because 
> the intersection of a point and a line is a point.
> This is better than my solution cuz generate_series actually has 
> issues when the number of points in a line is more than a few thousand 
> (probably due to internal storage and toast tables).
>
> So, your query would become:
> SELECT ST_BuildArea(geom) AS geom
> FROM
> (
>   -- Re-node the linear ring
>   SELECT St_Union(geom, St_Startpoint(geom)) AS geom
>   FROM
>   (
>      -- The exterior ring of your polygon
>      SELECT ST_ExteriorRing(
>        'POLYGON((
>            9.50351715087891 47.3943328857422,
>            9.50386047363281 47.3943328857422,
>            9.50351715087891 47.3943328857422,
>            9.50248718261719 47.3943328857422,
>            9.50214385986328 47.3939895629883,
>            9.50180053710938 47.3943328857422,
>            9.50145721435547 47.3939895629883,
>            9.50111389160156 47.3936462402344,
>            9.50145721435547 47.3936462402344,
>            9.50145721435547 47.3939895629883,
>            9.50214385986328 47.3939895629883,
>            9.50248718261719 47.3939895629883,
>            9.50386047363281 47.3943328857422,
>            9.50351715087891 47.3943328857422))'::geometry) AS geom
>   ) AS ring
> ) AS fixed_ring
>
> -- Kevin
>
> -------------
> Kevin Neufeld
> Software Developer
> Refractions Research Inc.
> 300-1207 Douglas St.
> Victoria, B.C., V8W 2E7
>
> Phone: (250) 383-3022
> Email: kneufeld at refractions.net
>
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-- 
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022