ST_Union behaviour

[Martin Davis]

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?
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> 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.
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> In these cases you should be using ST_UnaryUnion
> https://postgis.net/docs/ST_UnaryUnion.html
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> *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
>
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> Does someone know why ST_Union behaves differently with linestrings and
> polygons?
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> If a table contains linestrings that overlap at some points then ST_Union
> creates a collection of linestrings  that are splitted at intersections.
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> 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).
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> Thanks in advance.
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> Antonio
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