Merging linear overlay results?

[Daniel Baston]

I've opened a draft PR [1] with the change under discussion, so you can get
an idea of the changes requires to GEOS tests.

One thing I hadn't anticipated is that some GEOS clients (at least, PostGIS
and shapely) rely on unmerged outputs as a way of splitting a linear
geometry. (Is this the usage you refer to above, Martin?) Since the next
GEOS release will include a direct "split" operation, it _should_ be
possible for those implementation to just switch from GEOSDifference to
GEOSSplit.

Dan

[1] https://github.com/libgeos/geos/pull/1459

On Fri, Jun 26, 2026 at 4:11 PM Martin Davis <mtnclimb at gmail.com> wrote:

> The original idea behind leaving linear overlay results unmerged was to
> allow the user to choose whether they wanted to incur the overhead of line
> merging.  However, as the Polygon-Polygon example makes obvious, for some
> types of input this strategy produces non-intuitive results.  So it seems
> like a good idea to provide merging automatically.
>
> One downside is that then overlay cannot be used as a way to perform
> noding of sets of LineStrings.  A way of preserving this would be to merge
> only line segments which orginated in the same parent LineString.
> Actually, it's probably necessary to merge only segments which were
> contiguous in the parent.
>
> On Wed, May 20, 2026 at 7:00 AM Daniel Baston <dbaston at gmail.com> wrote:
>
>> Hi,
>>
>> I've noticed that the GEOS overlay code has the capability to merge
>> linear output geometries, using LineBuilder::addResultLinesMerged. However,
>> this capability has never been enabled. GEOS clients ([1], [2]) sometimes
>> end up calling GEOSLineMerge as a follow-up step to overlay, but this is
>> inefficient and cumbersome. (Among other things, GEOSLineMerge silently
>> drops any non-linear inputs.)
>>
>> Does it make sense to consider enabling this behavior? It would be
>> helpful for reconstructing CompoundCurves, which have no way to retain
>> their complex parentage information through the noding journey. I guess we
>> would need a GEOSUnionWithParams, GEOSIntersectionWithParams, etc...
>>
>> Dan
>>
>> [1]
>> https://github.com/qgis/QGIS/blob/b298119a5b09ae3903596ec2ffaf8aaff7ba78a3/src/core/geometry/qgsgeos.cpp#L1985
>> [2]
>> https://github.com/OSGeo/gdal/blob/ad495515b681bfb5f660f5de17e9ef1c2c8f153f/apps/gdalalg_vector_dissolve.cpp#L134
>>
>>
>>
>>
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