Clarification on ST_CoverageClean Parameters: snappingDistance and gapMaximumWidth

[Martin Davis]

And note that the correct signature

geometry ST_CoverageClean(geometry winset geom, float8 gapMaximumWidth = 0,
float8 snappingDistance = -1, text overlapMergeStrategy =
'MERGE_LONGEST_BORDER');

means that the best-practice pattern for using the function is to simply
specify the maximum gap width, leaving the snapping tolerance and overlap
merge strategy as defaulits:

CREATE TABLE  example1_c AS
  SELECT id,  ST_CoverageClean(geom, 1) OVER () AS GEOM
  FROM  example1;


On Tue, Oct 7, 2025 at 10:49 AM Paul Ramsey via postgis-users <
postgis-users at lists.osgeo.org> wrote:

> 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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