Points Along Lines

Draft: written, not yet checked by a person Operation · points-along-lines

Points placed along each line at a regular distance.

Pitfall. Engines differ on whether the first point is at the line's start, at the first full distance, or half a step in, and on whether the end gets a point.

Pitfall. Some take a fraction of the line's length or a number of points per unit length instead of a distance.

About this operation
Also called	Generate Points Along Lines, points along geometry, line sample, interpolate points, stationing
Group	`vector/geometry`
Checks	`crs.measure`
Pitfalls	2 listed above
Columbus tool	Not stated
Notes	Not stated
Status	active
Broader	Not stated
Narrower	Not stated
Related	`densify`

Parameters

Name	Type	Required	Default	Values	Note
`distance`	length	yes	Not stated	Not stated	Spacing along the line: a point at the start, then one every distance.
`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	point	Not stated

In each engine

Engine	Tool	Parameters	Differences
`grass`	`v.to.points`	`distance` → `dmax` `unit` always crs-units. dmax is in the map units of the GRASS project. `layer` → `input` `output` → `output`	Spaces the points evenly between vertices, both ends included, so a 95 m line at 10 m gets 11 points 9.5 m apart; with one more step than needed when the length is an exact multiple of dmax (a 100 m line at 10 m gets 12 points, 9.09 m apart). Keeps every vertex of the line and adds points between them so that none is further apart than dmax; flag i (set by this mapping) does the interpolation, without it only the vertices are output. Preset: `{"flag_i":true,"use":"vertex"}`
`postgis`	`st_lineinterpolatepoints`	`distance` Takes a fraction of the line's length: divide the distance by ST_Length of the line. `unit` always crs-units `layer` → `#1`	The first point is one step from the start; there is no point at the start. Returns one MultiPoint per line.
`qgis`	`native:pointsalonglines`	`distance` → `DISTANCE` `unit` always crs-units `layer` → `INPUT` `output` → `OUTPUT`	Starts at the line's start; the end gets a point only when it falls on a step; START_OFFSET and END_OFFSET leave a stretch at either end without points. Adds the distance along the line and the line's angle at each point as fields.
`sf`	`sf::st_line_sample`	`distance` density is points per unit of length, the inverse of the distance; n gives a number of points instead. `unit` always crs-units `layer` → `x`	With type regular the points sit half a step in from each end, not at the start. Returns one MULTIPOINT per line.