[postgis-users] Getting boundary of set of polygons

[Tom McCallum]

Thanks for all your replies, not quite there but got some ideas.


On Mon, 27 Jan 2014 13:51:26 -0000, Rémi Cura <remi.cura at gmail.com> wrote:

> You could use convex or concav envelop.
> You can also union the geometries (eventually simplifying by neg buffer
> then pos buffer)
> That's if you want precise boundary.
> If you just need bbox ...
> Cheers,
> Rémi-C
>
>
> 2014-01-27 Hugues François <hugues.francois at irstea.fr>
>
>> Hello,
>>
>> On my side, I often use
>> st_buildarea(st_exteriorring((st_dump(geom)).geom)). And if I want to  
>> keep
>> a single object for the original one, I union or collect them regarding
>> their gid.
>>
>> HTH
>>
>> Hugues.
>>
>>
>>
>>
>> -----Original Message-----
>> From: postgis-users-bounces at lists.osgeo.org [mailto:
>> postgis-users-bounces at lists.osgeo.org] On Behalf Of Tom McCallum
>> Sent: Monday, January 27, 2014 1:51 PM
>> To: PostGIS Users Discussion
>> Subject: Re: [postgis-users] Getting boundary of set of polygons
>>
>> St_MakePolygon does not take a multilinestring.
>> ST_BuildArea looked possible, but it gives me back lots of little areas
>> and not a single boundary for the whole set.
>>
>> It looks as though ST_Dump with ST_MakePolygon may be useful but am  
>> still
>> trying to get the right form something like
>>
>> ST_MakePolygon( ST_DUMP( ST_BOUNDARY( ST_COLLECT( wkb_boundary ) )  
>> .geom )
>>
>> But it does not like the .geom bit.
>>
>> Any suggestions?
>>
>> On Mon, 27 Jan 2014 12:35:51 -0000, Nicolas Ribot  
>> <nicolas.ribot at gmail.com>
>> wrote:
>>
>> > Hi,
>> >
>> > You should try st_makePolygon, or st_buildArea depending on the result
>> > you want (keeping holes or removing them).
>> >
>> > Nicolas
>> >
>> >
>> >
>> > On 27 January 2014 13:32, Tom McCallum <termcc at googlemail.com> wrote:
>> >
>> >> Hello all,
>> >>
>> >> I am trying to get the boundary of a geometry(polygon) field in
>> >> postgis (multiple polygons in this field).  I have this so far:
>> >>
>> >> ST_BOUNDARY(ST_COLLECT(wkb_geometry))
>> >>
>> >> which outputs MultilineString.  I need to convert this to a polygon.
>> >> I have tried all sorts of things from ST_DUMP to converting to text
>> >> and then trying ST_BdPolyFromText but got nothing to work so far.
>> >> ST_Convexhull gives me a polygon but unfortunately its not what I
>> >> want.
>> >>
>> >> Could someone let me know what I should be calling.
>> >>
>> >> Thanks
>> >>
>> >> Tom
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