Snap

Draft: written, not yet checked by a person Operation · snap

Moves vertices of each feature onto nearby vertices or edges of a reference layer, within a tolerance.

Pitfall. Engines differ on whether they snap to the reference's vertices only or also to its edges, and on whether they add vertices.

Pitfall. A tolerance larger than the features' own detail can collapse them or make them invalid.

About this operation
Also called	snap geometries, Snap (Editing), snap to layer, align vertices
Group	`vector/geometry`
Checks	`crs.agreement, crs.measure, geometry.valid`
Pitfalls	2 listed above
Columbus tool	Not stated
Notes	Not stated
Status	active
Broader	Not stated
Narrower	Not stated
Related	`simplify` `make-valid`

Parameters

Name	Type	Required	Default	Values	Note
`tolerance`	length	yes	Not stated	Not stated	Largest distance a vertex may move.
`unit`	enum	yes	Not stated	`m, ft, us-ft, km, mi`	Not stated

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

Engine	Tool	Parameters	Differences
`apache-sedona`	`sedona.sql.ST_Snap`	`tolerance` → `tolerance` `unit` always crs-units `layer` → `input` `reference` → `reference`	A Spark SQL function: arguments are columns or column names, run over a Spark DataFrame.
`geopandas`	`geopandas.GeoSeries.snap`	`tolerance` → `tolerance` `unit` always crs-units `reference` → `other`	other is one geometry or a GeoSeries matched row by row (align), not a whole reference layer.
`postgis`	`st_snap`	`tolerance` → `#3` `unit` always crs-units `layer` → `#1` `reference` → `#2`	One geometry against one reference geometry: ST_Collect the reference layer first.
`qgis`	`native:snapgeometries`	`tolerance` → `TOLERANCE` `unit` always crs-units `layer` → `INPUT` `reference` → `REFERENCE_LAYER` `output` → `OUTPUT`	BEHAVIOR (required; this mapping sets 0) chooses how: prefer nodes and add vertices (0), prefer closest point, nodes only, end points only, and more. Preset: `{"BEHAVIOR":0}`
`sf`	`sf::st_snap`	`tolerance` → `tolerance` `unit` always crs-units `layer` → `x` `reference` → `y`	None noted
`shapely`	`shapely.snap`	`tolerance` → `tolerance` `unit` always crs-units `layer` → `geometry` `reference` → `reference`	Snaps to the reference's vertices and edges (GEOS); one geometry against one reference geometry.
`terra`	`terra::snap`	`tolerance` → `tolerance` `unit` always crs-units `layer` → `x` `reference` → `y`	Without y, snaps the layer's own features to each other.