Nested holes

geometry

A hole containing an island (hole-in-hole-in-fill) violates simple-polygon assumptions; naive ring parity gets the interior/exterior classification backwards.

Nested holes illustration — ▪ Rendered from the real conversion/geometry code.

Detection

Ring nesting depth > 2 (exterior -> hole -> island -> hole...) detected via point-in-ring containment tests between rings.

Mitigation

Flatten to a single fill/hole pair per nesting level via GEOS/turf union
Validate with even-odd or nonzero winding rule before filling
Reject geometries with nesting depth > 3 pending manual review

Affected conversions

Polygon → H3 (center-contained) approximate
Polygon → H3 (fully-contained) conservative
Polygon → H3 (intersects) expansive