[postgis-users] return boundary

[Kevin Neufeld]

I think you're right ... there isn't a simple answer to your problem.

A complicated approach might be something like:
1. extract the boundary lines from your polygons.
2. convert the boundary lines into a collection of two-coordinate linestrings.
3. remove the closest pairs of linestrings from neighbouring polygons.
4. collect the lines and repolygonize.

-- Kevin

Travis Kirstine wrote:
> I don't think that the geom union would in combination with the
> boundary would attach the island to the mainland.  I don't think that
> there is a simple answer to my problem but I have attached a pic to
> help illustrate what I'm trying to accomplish.
> 
> Regards
> 
> 2008/11/21 Lee Meilleur <lee.meilleur at gis.leg.mn>:
>> Howabout ST_BOUNDARY(ST_UNION(the_geom))
>>
>> ________________________________________
>> From: postgis-users-bounces at postgis.refractions.net [postgis-users-bounces at postgis.refractions.net] On Behalf Of Burgholzer,Robert [rwburgholzer at deq.virginia.gov]
>> Sent: Friday, November 21, 2008 12:45 PM
>> To: PostGIS Users Discussion
>> Subject: RE: [postgis-users] return boundary
>>
>> So,
>> Do you need to iterate through the sub-polygons in your multi and get a
>> boundary of each?
>>
>> r.b.
>>
>> Robert W. Burgholzer
>> Surface Water Modeler
>> Office of Water Supply and Planning
>> Virginia Department of Environmental Quality
>> rwburgholzer at deq.virginia.gov
>> 804-698-4405
>> Open Source Modeling Tools:
>> http://sourceforge.net/projects/npsource/
>>
>> -----Original Message-----
>> From: postgis-users-bounces at postgis.refractions.net
>> [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
>> Travis Kirstine
>> Sent: Friday, November 21, 2008 10:51 AM
>> To: PostGIS Users Discussion
>> Subject: Re: [postgis-users] return boundary
>>
>> Thanks Obe
>> But I'm looking for something that returns the boundary of all the
>> selected features combined and attached.  Something that has the
>> abilty to take the group the features together like convex hull but
>> keep the detail of the of the edge like boundary.
>>
>> 2008/11/21 Obe, Regina <robe.dnd at cityofboston.gov>:
>>> How about ST_Boundary?
>>>
>> http://postgis.refractions.net/documentation/manual-svn/ST_Boundary.html
>>> -----Original Message-----
>>> From: postgis-users-bounces at postgis.refractions.net
>>> [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
>>> Travis Kirstine
>>> Sent: Friday, November 21, 2008 10:18 AM
>>> To: PostGIS Users Discussion
>>> Subject: [postgis-users] return boundary
>>>
>>> I have a multipolygon that has consists mainland and island polygons.
>>> I need to generate a single polygon / line that represent the boundary
>>> of the multipolygon as a whole, something like a convex hull but
>>> keeping the detail of the outer rings and joining the islands to the
>>> mainland.  I have been try different combos of convex hull, buffer and
>>> difference but can't get what I'm looking for.
>>>
>>> Any thoughts would be greatly appreciated
>>>
>>> Regards
>>>
>>> --
>>> Travis K.
>>>
>>> Toronto, Canada
>>> ------------------------------------------------------------
>>> "She knows there's no success like failure
>>> And that failure's no success at all."
>>> -Bob Dylan-
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>>
>>
>> --
>> Travis K.
>>
>> Toronto, Canada
>> ------------------------------------------------------------
>> "She knows there's no success like failure
>> And that failure's no success at all."
>> -Bob Dylan-
>> ------------------------------------------------------------
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