IDW Interpolation

Draft: written, not yet checked by a person Operation · idw-interpolation

A raster surface interpolated from point values, each cell a distance-weighted average of nearby points.

Pitfall. Engines differ on which points each cell uses (all of them, a search radius, or the nearest N), so surfaces differ away from the points.

Pitfall. IDW never goes above the highest or below the lowest input value, so it flattens peaks and pits.

About this operation
Also called	inverse distance weighted, IDW, inverse distance to a power, surface from points
Group	`conversion/vector-to-raster`
Checks	`crs.present, raster.nodata`
Pitfalls	2 listed above
Columbus tool	Not stated
Notes	Not stated
Status	active
Broader	Not stated
Narrower	Not stated
Related	`rasterize`

Parameters

Name	Type	Required	Default	Values	Note
`field`	field	yes	Not stated	Not stated	The attribute holding the values to interpolate.
`resolution`	number	yes	Not stated	Not stated	Cell size, in the layer's CRS units.
`power`	number	no	`2`	Not stated	Higher powers give nearer points more weight.

Inputs and outputs

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

In each engine

Engine	Tool	Parameters	Differences
`gdal`	`gdal vector grid invdist`	`field` → `zfield` `resolution` resolution (x and y) must come with extent (xmin, ymin, xmax, ymax); give the grid by both, or by size. `power` → `power` `layer` → `input` `output` → `output`	Uses every point by default; radius, max-points and min-points limit the search. Cells with too few points get nodata (default 0).
`grass`	`v.surf.idw`	`field` → `column` `resolution` Set by the computational region (g.region res=...). `power` → `power` `layer` → `input` `output` → `output`	Uses the nearest npoints points (default 12).
`qgis`	`gdal:gridinversedistance`	`field` → `Z_FIELD` `resolution` No cell size argument; set the output size or resolution with -tr in EXTRA. `power` → `POWER` `layer` → `INPUT` `output` → `OUTPUT`	None noted
`qgis`	`qgis:idwinterpolation`	`field` Encoded in INTERPOLATION_DATA. `resolution` → `PIXEL_SIZE` `power` → `DISTANCE_COEFFICIENT` `layer` Layers, fields and types go in INTERPOLATION_DATA as one encoded string. `output` → `OUTPUT`	None noted
`terra`	`terra::interpIDW`	`field` → `field` `resolution` x is a template raster on the target grid. `power` → `power` `layer` → `y`	radius is required; cells with no point within it get fill (default NA).
`whiteboxtools`	`IdwInterpolation`	`field` → `field` `resolution` → `cell_size` `power` → `weight` `layer` → `input` `output` → `output`	Without radius or min_points every cell came out as nodata in the equivalence tests; min_points must not exceed the number of points. base gives a raster whose grid to match.
`xarray-spatial`	`xrspatial.idw`	`field` z takes the values. `resolution` template is a DataArray on the target grid. `power` → `power` `layer` x, y and z take the points' coordinates and values as arrays.	k limits each cell to the k nearest points.