Measure Length

Draft: written, not yet checked by a person Operation · measure-length

The length of each line, as a number or a new attribute.

Pitfall. In a geographic CRS, planar engines return degrees.

Pitfall. For polygons, some functions return the perimeter and others 0 (PostGIS ST_Length); use a perimeter function for outlines.

About this operation
Also called	Calculate Geometry (length), line length, shape length, perimeter
Group	`vector/measure`
Checks	`crs.measure`
Pitfalls	2 listed above
Columbus tool	Not stated
Notes	Not stated
Status	active
Broader	Not stated
Narrower	Not stated
Related	`measure-area`

Parameters

Name	Type	Required	Default	Values	Note
`unit`	enum	no	`m`	`m, ft, us-ft, km, mi`	Not stated

Inputs and outputs

Direction	Name	Type	Geometry	Note
Input	`layer`	vector	line	Not stated
Output	`output`	any	Not stated	The lengths, or the layer with a length attribute.

In each engine

Engine	Tool	Parameters	Differences
`apache-sedona`	`sedona.sql.ST_Length`	`unit` always crs-units. CRS units (metres in a metric CRS); convert afterwards for other units. `layer` → `geometry`	A Spark SQL function: arguments are columns or column names, run over a Spark DataFrame.
`geopandas`	`geopandas.GeoSeries.length`	`unit` always crs-units. CRS units (metres in a metric CRS); convert afterwards for other units.	A property of the GeoSeries; for polygons, the perimeter.
`grass`	`v.to.db`	`unit` → `units` : m = meters; ft = feet; km = kilometers; mi = miles. not supported: us-ft `layer` → `map`	Writes into the map's own attribute table (this mapping names the column length). Preset: `{"option":"length","columns":"length"}`
`postgis`	`st_length`	`unit` always crs-units. CRS units (metres in a metric CRS); convert afterwards for other units. `layer` → `#1`	Returns 0 for polygons (ST_Perimeter measures outlines); on geography, metres on the spheroid.
`qgis`	`native:exportaddgeometrycolumns`	`unit` always crs-units. CRS units (metres in a metric CRS); METHOD 2 measures on the ellipsoid. `layer` → `INPUT` `output` → `OUTPUT`	Adds a length field to lines (area and perimeter to polygons).
`sf`	`sf::st_length`	`unit` always crs-units. CRS units (metres in a metric CRS); convert afterwards for other units. `layer` → `x`	Returns a units object; 0 for polygons (st_perimeter measures outlines).
`shapely`	`shapely.length`	`unit` always crs-units. CRS units (metres in a metric CRS); convert afterwards for other units. `layer` → `geometry`	For polygons, returns the perimeter.
`terra`	`terra::perim`	`unit` always crs-units. CRS units (metres in a metric CRS); convert afterwards for other units. `layer` → `x`	For polygons, the perimeter.
`turf-js`	`turf.length`	`unit` → `options.units` : m = meters; ft = feet; km = kilometers; mi = miles. not supported: us-ft `layer` → `geojson`	units defaults to kilometers, so pass the unit even when it is the canonical default (metres); one total for all features. Turf works on GeoJSON in longitude/latitude (WGS 84); data in another CRS must be reprojected first.