Contour

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

Lines joining cells of equal value in a raster, at a regular interval.

Pitfall. Contours are interpolated between cell centres, so they are only as precise as the cell size; engines differ on whether lines stop at the outermost cell centres (GRASS) or run to the raster's edge (GDAL).

Pitfall. Some engines smooth or simplify the lines by default, which changes their lengths and positions slightly.

About this operation
Also called	contours, contour lines, isolines, Contour List
Group	`raster/surface`
Checks	`raster.nodata, geometry.valid`
Pitfalls	2 listed above
Columbus tool	Not stated
Notes	Not stated
Status	active
Broader	Not stated
Narrower	Not stated
Related	`slope` `hillshade`

Parameters

Name	Type	Required	Default	Values	Note
`interval`	number	yes	Not stated	Not stated	Spacing between levels, in the raster's value units.
`base`	number	no	`0`	Not stated	Levels are base plus multiples of interval.
`field`	string	no	Not stated	Not stated	Name of the attribute that holds each line's level.
`band`	integer	no	`1`	Not stated	Not stated

Inputs and outputs

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

In each engine

Engine	Tool	Parameters	Differences
`gdal`	`gdal raster contour`	`interval` → `interval` `base` → `offset` `field` → `elevation-name` `band` → `band` `raster` → `input` `output` → `output`	None noted
`grass`	`r.contour`	`interval` → `step` `base` No offset; minlevel and maxlevel limit the levels, or levels lists them. `field` The level goes in the attribute table's level column. `raster` → `input` `output` → `output`	None noted
`postgis-raster`	`st_contour`	`interval` → `level_interval` `base` → `level_base` `field` Returns rows of (geom, id, value); value is the level. `band` → `bandnumber` `raster` → `rast`	None noted
`qgis`	`gdal:contour`	`interval` → `INTERVAL` `base` → `OFFSET` `field` → `FIELD_NAME` `band` → `BAND` `raster` → `INPUT` `output` → `OUTPUT`	None noted
`terra`	`terra::as.contour`	`interval` Give levels (or nlevels), passed through ... to grDevices::contourLines. `raster` → `x`	Uses at most maxcells cells (default 100,000), sampling larger rasters.
`whiteboxtools`	`ContoursFromRaster`	`interval` → `interval` `base` → `base` `raster` → `input` `output` → `output`	Smooths (smooth, default 9) and simplifies (tolerance, default 10) the lines by default.
`xarray-spatial`	`xrspatial.contours`	`interval` Give levels: a list of values (default n_levels evenly spaced levels). `raster` → `agg`	Returns (level, coordinates) pairs, or a GeoDataFrame with return_type = 'geopandas'.