Mosaic

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

Combines several rasters into one covering all of them.

Pitfall. Where inputs overlap, engines differ on which value wins (first, last, mean); check before mosaicking overlapping tiles.

Pitfall. Inputs with different cell sizes or CRSs are resampled or refused depending on the engine.

About this operation
Also called	Mosaic To New Raster, merge rasters, gdal_merge, stitch tiles, virtual mosaic
Group	`raster/combine`
Checks	`crs.agreement, raster.nodata`
Pitfalls	2 listed above
Columbus tool	Not stated
Notes	Not stated
Status	active
Broader	Not stated
Narrower	Not stated
Related	`merge-layers`

Parameters

Not stated

Inputs and outputs

Direction	Name	Type	Geometry	Note
Input	`rasters`	raster (several)	Not stated	Not stated
Output	`output`	raster	Not stated	Not stated

In each engine

Engine	Tool	Parameters	Differences
`gdal`	`gdal raster mosaic`	`rasters` → `input` `output` → `output`	Builds a virtual mosaic (VRT) unless the output is another format.
`grass`	`r.patch`	`rasters` → `input` `output` → `output`	The first map listed has priority; later maps only fill its no-data cells. The output covers the computational region.
`postgis-raster`	`st_union`	`rasters` → `#1`	An aggregate over raster rows; uniontype (LAST by default) chooses the value where tiles overlap.
`qgis`	`gdal:merge`	`rasters` → `INPUT` `output` → `OUTPUT`	Runs gdal_merge; where inputs overlap the last one in the list is copied over earlier ones.
`rasterio`	`rasterio.merge.merge`	`rasters` → `sources`	Returns an array and its transform; method chooses which value wins where inputs overlap (default first).
`rioxarray`	`rioxarray.merge.merge_arrays`	`rasters` → `dataarrays`	None noted
`terra`	`terra::merge`	`rasters` → `x` : The rasters go in x, y and further arguments, or a SpatRasterCollection in x.	Where inputs overlap, the first raster's values are kept (first = TRUE).
`terra`	`terra::mosaic`	`rasters` → `x` : The rasters go in x, y and further arguments, or a SpatRasterCollection in x.	Where inputs overlap, fun combines the values (default mean).
`whiteboxtools`	`Mosaic`	`rasters` → `inputs` `output` → `output`	In the equivalence tests the output grid was one row and one column short of the inputs' combined extent (two 10 by 20 halves gave 19 by 19).
`xarray-spatial`	`xrspatial.merge`	`rasters` → `rasters`	strategy chooses which value wins where inputs overlap (default first); inputs in other CRSs are reprojected.