[postgis-users] Getting boundary of set of polygons

[Rémi Cura]

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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