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 <[email protected]> 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 >
