Union

Draft: written, not yet checked by a person Operation · union-overlay

Splits two polygon layers where they overlap and keeps every piece, with the attributes of whichever layers cover it.

Pitfall. Not the same as merging shapes into one (that is dissolve, which many SQL and Python libraries call union).

About this operation
Also called	overlay union, Union (Analysis), identity of both layers
Group	`vector/overlay`
Checks	`crs.agreement, geometry.valid, extent.plausible`
Pitfalls	1 listed above
Columbus tool	Not stated
Notes	Not stated
Status	active
Broader	Not stated
Narrower	Not stated
Related	`intersect` `symmetric-difference` `dissolve`

Parameters

Not stated

Direction	Name	Type	Geometry	Note
Input	`a`	vector	polygon	Not stated
Input	`b`	vector	polygon	Not stated
Output	`output`	vector	polygon	Not stated

Engine	Tool	Parameters	Differences
`gdal`	`gdal vector layer-algebra`	`a` → `input` `b` → `method` `output` → `output`	None noted Preset: `{"operation":"union"}`
`grass`	`v.overlay`	`a` → `ainput` `b` → `binput` `output` → `output`	The b input must be areas. Preset: `{"operator":"or"}`
`qgis`	`native:union`	`a` → `INPUT` `b` → `OVERLAY` `output` → `OUTPUT`	None noted
`terra`	`terra::union`	`a` → `x` `b` → `y`	With one SpatVector it splits its own overlapping polygons.
`whiteboxtools`	`Union`	`a` → `input` `b` → `overlay` `output` → `output`	None noted