[postgis-users] Call for Semantics (ST_CollectionTypes())

[Stephen Woodbridge]

I can confirm this is a useful use case. I ran into the same issues 
doing soils analysis. I think a filter to extract the items you want 
would be very useful.

-Steve W

Chris Hermansen wrote:
> As someone accustomed to doing my geoprocessing in ArcInfo, one of the
> first things I was looking for in PostGIS was the ability to do:
> 
>     INTERSECT conserv_area forest cons_for POLY 1.0 NOJOIN
> 
> I guess I probably naively tried something like:
> 
>     CREATE TABLE cons_for AS
>     SELECT
> 
>         conserv_area_id,
>         forest_id,
>         st_intersection(conservation_areas, forest) the_geom
> 
>     FROM conservation_areas, forest
> 
> At that point, I must have wondered about what exactly that stuff was in
> the_geom in the new table
> 
> Maybe it's not too much of a stretch to wish for
> 
>     SELECT
>     st_just_gimme_the_polygons(st_intersection(conservation_areas,forest))
> 
> 
> Paul Ramsey wrote:
>> Well, right now, how do I turn a column of MULTIPOLYGON goemetries
>> into a single collected MULTIPOLYGON? I have to rather inefficiently
>> explode them, then re-collect them. Kind of a trivial example.
>>
>> More fiddly for people to currently solve, the ST_Intersection()
>> function can spit out heterogeneous GEOMETRYCOLLETIONS as a result.
>> And usually, the end-user just wants one component of that... if
>> intersecting lines on polygons, they just want lines, and don't care
>> about any points. Or if doing poly-on-poly, they don't care about any
>> lines or points that fall out the bottom, as occasionally happens.
>>
>> P.
>>
>>
>> On Wed, Jan 28, 2009 at 4:40 PM, Mark Cave-Ayland
>> <mark.cave-ayland at siriusit.co.uk> wrote:
>>   
>>> Paul Ramsey wrote:
>>>
>>>     
>>>> I would like to add a function that takes an anonymous
>>>> GEOMETRYCOLLECTION and returns something more structured:
>>>> - a GEOMETRYCOLLECTION where the first element is a MULTLIPOLYGON, the
>>>> second a MULTILINESTRING and the third a MULTIPOINT?
>>>> - a tuple, with elements 'polygons', 'linestrings', 'points', each
>>>> with one multi* of each?
>>>> - a tuple, with elements 'polygons', 'linestrings', 'point', each with
>>>> an array of singletons?
>>>>
>>>> The key here is to provide a useful flat structure to pull info out
>>>> of. Some information in the GEOMETRYCOLLECTION *will be lost*, (but
>>>> that seems an acceptable price)
>>>>
>>>> Another approach, which might be simpler for users to grok would be:
>>>>
>>>> ST_CollectionAsMultiPolygon() returns just the polygon elements of a
>>>> collection.
>>>> ... etc ...
>>>>
>>>> Thoughts?
>>>>
>>>> P.
>>>>       
>>> Hi Paul,
>>>
>>> I'm still struggling to get an idea of exactly what these proposed functions
>>> are trying to achieve. Can you show some potential problems/SQL statements
>>> showing how you would envisage these functions being used?
>>>
>>>
>>> ATB,
>>>
>>> Mark.
>>>
>>> --
>>> Mark Cave-Ayland
>>> Sirius Corporation - The Open Source Experts
>>> http://www.siriusit.co.uk
>>> T: +44 870 608 0063
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>>>
>>>     
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