Cases where ST_Overlaps(ST_Difference(a,b),b) = TRUE

[dannytoone]

POSTGIS="3.4.1 ca035b9" [EXTENSION] PGSQL="160" GEOS="3.12.1-CAPI-1.18.1" PROJ="9.2.1 NETWORK_ENABLED=OFF URL_ENDPOINT=https://cdn.proj.org USER_WRITABLE_DIRECTORY=/Users/DToone2/Library/Application Support/proj DATABASE_PATH=/Applications/Postgres.app/Contents/Versions/16/share/proj/proj.db" LIBXML="2.11.7" LIBJSON="0.17" LIBPROTOBUF="1.4.1" WAGYU="0.5.0 (Internal)"

I'm having a tough time thinking through the relational geometry of this, and I can't help but think this is a bug. If b is cut out from a, then the result should intersect with b along the shared boundary, but it shouldn't overlap. A resulting overlap would indicate that the ST_Difference was not complete. I've recently found several cases where ST_Overlaps(ST_Difference(a,b),b) = TRUE, and I thought I'd file a bug report to make sure.

[dannytoone]

pg_dump of table with 25 rows that meet bug conditiojn

[mdavis]

This can be due to these things:

• (most likely) The results of overlay operations (such a difference) are subject to numerical rounding, which means that the results might not exactly match the input linework. This can cause spatial predicates to be inconsistent with expected results

• (very rare) a bug in the overlay algorithm sometimes causes outputs to be very different from what is expected. This would show up as a large difference in area from the expected value.