Clarification on ST_CoverageClean Parameters: snappingDistance and gapMaximumWidth

[Paul Ramsey]

If I told you there is an error in the documentation and that the actual
signature is this, would that allay your concerns?

ST_CoverageClean (geom geometry, gapMaximumWidth float8 default 0.0,
snappingDistance float8 default -1.0, overlapMergeStrategy text default '
MERGE_LONGEST_BORDER')

P.

On Tue, Sep 23, 2025 at 5:25 AM Douglas Fan <douglas.mhfan at gmail.com> wrote:

> Dear PostGIS Developers and Users,
>
> First of all, thank you for the development of the new ST_CoverageClean
> function in PostGIS 3.6.0. It’s a fantastic addition that has already
> helped a lot in my work with polygonal coverages. I really appreciate the
> effort that went into making this tool available.
> While testing the function, I’ve encountered some behaviors that I’d like
> to better understand, particularly regarding the snappingDistance and
> gapMaximumWidth parameters.
>
> From the documentation, I understand that:
> 1. snappingDistance controls vertex snapping, with -1 applying an
> automatic distance, and 0.0 disabling snapping.
> 2. gapMaximumWidth closes gaps smaller than the specified tolerance.
>
> However, during testing with various combinations (e.g., snappingDistance
> set to -1, 0.0, 1, 2 and gapMaximumWidth set to 0, 1, 2), I noticed:
> 1. Even when snappingDistance is explicitly set to 0.0, small sliver
> vertices still appear to be snapped or altered when gapMaximumWidth is
> greater than 0.
> 2. Slivers that are thinner than the gapMaximumWidth are removed, even
> when they are not actual gaps or overlaps.
>
> This behavior seems counterintuitive, as I expected no snapping to occur
> with snappingDistance = 0.0. Could this be due to internal gap cleaning
> logic that also affects vertex positions? Or is there an implicit snapping
> step tied to gapMaximumWidth?
>
> I’d be grateful for any insights into:
>
> 1. How do these two parameters interact?
> 2. Best practices or recommended workflows for cleaning polygonal
> coverages with minimal geometry distortion.
> 3. Any known edge cases or limitations when using ST_CoverageClean.
> 4. Whether gapMaximumWidth also triggers geometry simplification or sliver
> removal beyond gap closing.
>
> Thanks in advance for your help and for the continued development of
> PostGIS.
>
> Best regards,
> Man Ho Fan
>
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