Offset Curve

Draft: written, not yet checked by a person Operation · offset-curve

A line running parallel to each input line at a given distance to one side.

Pitfall. Engines differ on which side a positive distance goes; check the direction of the line and the engine's convention.

Pitfall. At bends, joins (round, mitre, bevel) change the line's length; tight curves can make the offset line loop or vanish.

Pitfall. In a geographic CRS the distance is in degrees.

About this operation
Also called	Offset line, parallel line, Copy Parallel, offset curve, line offset
Group	`vector/geometry`
Checks	`crs.measure, geometry.valid`
Pitfalls	3 listed above
Columbus tool	Not stated
Notes	Not stated
Status	active
Broader	Not stated
Narrower	Not stated
Related	`buffer`

Parameters

Name	Type	Required	Default	Values	Note
`distance`	length	yes	Not stated	Not stated	Positive to the left of the line's direction, negative to the right.
`unit`	enum	yes	Not stated	`m, ft, us-ft, km, mi`	Not stated

Inputs and outputs

Direction	Name	Type	Geometry	Note
Input	`layer`	vector	line	Not stated
Output	`output`	vector	line	Not stated

In each engine

Engine	Tool	Parameters	Differences
`apache-sedona`	`sedona.sql.ST_OffsetCurve`	`distance` → `distance` `unit` always crs-units `layer` → `geometry`	A Spark SQL function: arguments are columns or column names, run over a Spark DataFrame.
`geopandas`	`geopandas.GeoSeries.offset_curve`	`distance` → `distance` `unit` always crs-units	join_style (round, mitre, bevel) and quad_segs shape the bends.
`grass`	`v.parallel`	`distance` → `distance` `unit` always crs-units. The distance is in the map units of the GRASS project. `layer` → `input` `output` → `output`	GRASS 8.5's v.parallel returns a line of one vertex for a straight two-vertex line; lines of three or more vertices come out right. The distance must be positive: side chooses left, right (the default) or both; this mapping sets side = left to match a positive canonical distance. flag r makes rounded corners; flag b a buffer-like loop around the line. Preset: `{"side":"left"}`
`postgis`	`st_offsetcurve`	`distance` → `#2` `unit` always crs-units `layer` → `#1`	params takes 'quad_segs=N join=round\|mitre\|bevel mitre_limit=N' as text.
`qgis`	`native:offsetline`	`distance` → `DISTANCE` `unit` always crs-units `layer` → `INPUT` `output` → `OUTPUT`	JOIN_STYLE 0 round, 1 miter, 2 bevel; DISTANCE can be data-defined.
`shapely`	`shapely.offset_curve`	`distance` → `distance` `unit` always crs-units `layer` → `geometry`	join_style (round, mitre, bevel) and quad_segs shape the bends.
`turf-js`	`turf.lineOffset`	`distance` → `distance` `unit` → `options.units` : m = meters; ft = feet; km = kilometers; mi = miles. not supported: us-ft `layer` → `geojson`	A positive distance offsets to the right of the line's direction, the opposite of the canonical convention; negate it. units defaults to kilometers. Turf works on GeoJSON in longitude/latitude (WGS 84); data in another CRS must be reprojected first.