ST_Union behaviour

[Antonio Valanzano]

Thanks for the explanation.

Il Mar 24 Dic 2024, 18:32 Martin Davis <mtnclimb at gmail.com> ha scritto:

> The behaviour of ST_Union with polygons follows the mathematical concept
> of union in point-set topology.  It produces as few polygons as possible
> which contain all the points of the input polygons.
>
> For LineStrings, they obviously can't "fork", so they are cut at
> intersections.  And actually ST_Union goes a bit further and cuts the
> linework at every node point.  This was a design decision from way back.
> It avoids the cost of merging contiguous lines (which can be done post
> facto with ST_LineMerge).
>
> What you want is often called "overlay".  Currently the best way to do it
> is via the technique described in Paul's blog post, or via PostGIS Topology
> (which might be more scalable but slower).
>
> On Mon, Dec 23, 2024 at 9:56 AM Regina Obe <lr at pcorp.us> wrote:
>
>> Are you unioning one geometry or many?
>>
>>
>>
>> The only reason I can think of why ST_Union would return unchanged
>> overlapping polygons is if you fed it a geometry collection or invalid
>> multipolygon with overlapping polygons.
>>
>>
>>
>> In these cases you should be using ST_UnaryUnion
>> https://postgis.net/docs/ST_UnaryUnion.html
>>
>>
>>
>>
>>
>>
>>
>> *From:* Antonio Valanzano <anvalanz at gmail.com>
>> *Sent:* Monday, December 23, 2024 3:47 AM
>> *To:* postgis-users at lists.osgeo.org
>> *Subject:* ST_Union behaviour
>>
>>
>>
>> Does someone know why ST_Union behaves differently with linestrings and
>> polygons?
>>
>>
>>
>> If a table contains linestrings that overlap at some points then ST_Union
>> creates a collection of linestrings  that are splitted at intersections.
>>
>>
>>
>> If a table contains polygons with overlaps then ST_Union creates a
>> collection of polygons that are still overlapped and are not splitted (the
>> result contains only the original polygons).
>>
>>
>>
>> Thanks in advance.
>>
>>
>>
>> Antonio
>>
>>
>>
>>
>>
>
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