ST_NDims

What is ST_NDims?

ST_NDims is a PostGIS function that returns the coordinate dimension of a geometry as an integer: 2 for XY, 3 for XYZ or XYM, and 4 for XYZM.

ST_NDims(geometry g) → integer

Synonym of ST_CoordDim — PostGIS-native naming versus the SQL/MM-standard alias. Either works interchangeably.

When would you use ST_NDims?

Use ST_NDims to inspect or enforce dimensionality across a dataset, for instance to route 3D geometries to Z-aware operations and 2D geometries to the 2D path:

1SELECT id,
2       CASE WHEN ST_NDims(geom) >= 3
3            THEN ST_3DLength(geom)
4            ELSE ST_Length(geom)
5       END AS length
6FROM tracks;

Also useful for data audits: SELECT ST_NDims(geom), COUNT(*) FROM features GROUP BY 1;

FAQs

ST_NDims vs ST_CoordDim — which should I use?

Equivalent functions. ST_CoordDim is the SQL/MM-standard name; ST_NDims is the PostGIS-native name. Pick one and use it consistently.

How does ST_NDims differ from ST_Dimension?

ST_Dimension returns the topological dimension (0 point, 1 line, 2 polygon) independent of Z/M. ST_NDims returns the coordinate count per vertex (2, 3, or 4).

Can ST_NDims tell me whether a geometry has Z or M?

Not directly — both POINT Z and POINT M return 3. Use ST_Zmflag to distinguish them.

Does ST_NDims inspect every vertex?

No — dimensionality is a type-level property stored in the geometry header, so this is a constant-time lookup regardless of vertex count.