Mastering PostGIS

Checking for validity

There are three PostGIS functions for checking validity. The first one is the ST_IsValid function. It behaves exactly like ST_IsSimple: it accepts a geometry and returns a Boolean indicating whether the input is valid or not.

SELECT * FROM data_import.ne_110m_land WHERE ST_IsValid(geom) =  FALSE; 

But what's exactly wrong with returned geometries? PostGIS can return the details of a validity check. Those can be returned in two variants: as a human readable summary or a compound type that includes the summary and location in geometry format. The first variant is named ST_IsValidReason:

SELECT ST_IsValidReason(geom) FROM data_import.ne_110m_land WHERE ST_IsValid(geom) =  FALSE;  

It returns a summary of test results: