ST_Length2D

What is ST_Length2D?

ST_Length2D returns the 2D length of a linear geometry. It is effectively a name alias of ST_Length for geometry input and exists primarily for API symmetry with ST_3DLength.

ST_Length2D(geometry a_2dlinestring) → float

The result ignores any Z values in the input and is expressed in the CRS units of the geometry.

When would you use ST_Length2D?

Use ST_Length2D when you want to be explicit that your length computation is the planimetric (map-view) length, as opposed to the slope length. This is often the right choice for cartographic measurements on 3D data — for example, the on-map length of a pipeline regardless of elevation changes. It pairs naturally with ST_3DLength to compute a slope-over-plan ratio.

1-- Slope ratio of a contoured cable run
2SELECT id,
3       ST_3DLength(geom) / NULLIF(ST_Length2D(geom), 0) AS slope_factor
4FROM cables;

FAQs

How is ST_Length2D different from ST_Length?

For geometry input they are equivalent. ST_Length2D exists to make the 2D intent explicit, especially alongside ST_3DLength. Only ST_Length accepts geography.

What units does it return?

The CRS units of the input — degrees for EPSG:4326, metres for most projected CRSs. There is no geography overload.

Does it consider Z coordinates?

No. ST_Length2D explicitly ignores Z. Use ST_3DLength if you need the true slope length including elevation.

What does it return for non-linear input?

Zero. Polygons, points, and collections without linear components return 0. For polygon boundary length use ST_Perimeter2D.