Elevation and slope (DEM)

type = "dem" reads a digital elevation model over the grid and gives elevation or slope as a raster source, with a checksum and provenance. The usual next step is a zonal operator such as mean.

[grid]
name = "area"
bbox = [-54.842, -3.587, -54.459, -3.168]
resolution = 500

[[source]]
id      = "slope"
type    = "dem"
dem     = "srtm"            # srtm | copernicus | topodata
product = "slope_deg"       # elevation | slope_deg | slope_pct

[[derive]]
target   = "mean_slope"
source   = "slope"
operator = "mean"

Fields

Field	Meaning
dem	srtm, copernicus or topodata. Give this or tiles.
tiles	Your own GeoTIFFs (paths relative to the pipeline file, or URLs GDAL reads) instead of a download.
product	elevation (m, default), slope_deg or slope_pct.
margin	Degrees added around the grid (default 0.02): a slope needs the neighbours of the edge pixels.
resolution	Metres of the grid on which the slope is computed (default 30).
cache	Folder for downloaded TOPODATA sheets (default $DISSCUBE_CACHE/dem or ~/.cache/disscube/dem).

The three DEMs

dem	What	How it is read
srtm	SRTM 1 arc-second, February 2000 (C-band radar), as republished in the AWS Terrain Tiles. No account.	Only the window over the grid, through HTTP range requests.
copernicus	Copernicus GLO-30, 2011–2015 (X-band). Free licence, © DLR/Airbus.	Only the window, from the public Cloud-Optimized GeoTIFFs on AWS.
topodata	INPE's TOPODATA: SRTM refined to 1 arc-second by kriging; Brazil only.	The 1° × 1.5° ZIP sheet is downloaded once into the cache.

For slope, prefer SRTM or TOPODATA. Copernicus GLO-30 is a surface model: it includes the forest canopy, so every forest/pasture edge looks like a ~30 m cliff and the slope is overestimated exactly where deforestation happens. DisSCube logs a warning when you ask for a Copernicus slope. In the Lab15 reconstruction, moving from Copernicus to SRTM raised the correlation of the mean slope with the original from 0.82 to 0.90.

How the slope is made

An operator aggregates the pixels of a cell and does not see their neighbours, so the slope is computed in the source: the window is resampled (bilinear) to the UTM zone of the grid's centre at resolution metres, so the slope is metric, and taken by central differences (numpy.gradient). This is not Horn's 3 × 3 formula, so values differ slightly from gdaldem slope. Voids (SRTM marks them −32768) and anything outside −1 000 to 9 000 m become NaN, and the slope is NaN around them.

The GeoTIFF is float32 with NaN as nodata: elevation in EPSG:4326, slopes in UTM. raw/<id>.provenance.json records the DEM, its licence, the tiles, the window, the processing and the checksum.

Tiles are not always there

SRTM and Copernicus have no tile over the open sea. A tile that cannot be opened is skipped with a warning; if none can be read, the run stops with the reasons. Check the warning if the grid reaches the coast.