ST_Zmflag

What is ST_Zmflag?

ST_Zmflag is a PostGIS function that returns a small integer indicating which optional dimensions a geometry carries:

ST_Zmflag(geometry g) → smallint

• 0 = 2D (X, Y only)
• 1 = 2D with M (X, Y, M)
• 2 = 3D with Z (X, Y, Z)
• 3 = 4D with both Z and M (X, Y, Z, M)

More informative than ST_NDims or ST_CoordDim, which cannot distinguish a POINT M from a POINT Z (both return 3).

When would you use ST_Zmflag?

Use ST_Zmflag when you need to distinguish Z-only, M-only, Z+M, and 2D geometries — for example when routing to Z-aware versus M-aware processing paths:

1SELECT id,
2       CASE ST_Zmflag(geom)
3         WHEN 0 THEN '2D'
4         WHEN 1 THEN 'XYM'
5         WHEN 2 THEN 'XYZ'
6         WHEN 3 THEN 'XYZM'
7       END AS dim_kind
8FROM features;

Also useful for data audits and schema-validation queries: confirming an entire table holds 3D-Z data, for instance.

FAQs

Why not just use ST_NDims?

ST_NDims returns 3 for both POINT Z and POINT M and cannot distinguish them. ST_Zmflag gives you the exact combination.

What does 2D mean here?

No Z, no M. Standard X/Y geometry.

Can the flag vary per vertex?

No — dimensionality is a type-level property, constant for all vertices of a given geometry.

How do I force a specific dimensionality?

Use ST_Force2D, ST_Force3D (adds Z=0), ST_Force3DM, ST_Force4D, or ST_ForceDimension variants to coerce input into a desired dimensionality.