Validity
For a polygon or multipolygon, when all of the following are true:.
For a polygon or multipolygon, when all of the following are true:

It is closed; its start point is the same as its end point.

Its boundary is a set of linestrings.

No two linestrings in the boundary cross. The linestrings in the boundary may touch at a point but they cannot cross.

Any polygons in the interior must be completely contained; they cannot touch the boundary of the exterior polygon except at a vertex.
Valid polygons:
Invalid polygon: