Slope and flow direction — Task
Implementation: Slope and flow direction from DHM (Recipe - QGIS). Data: the DHM terrain model through the DHM WCS — see WCS for why it must be a coverage, not a WMS picture.
Slope and flow direction is the Task of deriving, from a terrain model, how steep the ground is and which way water on the surface will run. Slope answers questions of accessibility, erosion risk and building suitability; flow direction is the first step of every surface-water analysis (catchments, flow paths, accumulation).
Used in examples: hydrologically corrected terrain models and Hydrological modelling (with the problem of bridges in the terrain model). Domain Examples bring in the domain knowledge (hydrology, planning) that decides which choices below matter; this Task stays domain-neutral.
In workflow terms it produces derived layers from primary elevation data: the results are only as good as the terrain model and the choices made below.
Task Schema (tool-agnostic)
- Choose the elevation source (decision point) — a terrain model (DTM, the bare ground), not a surface model (DSM): water does not flow over tree crowns and roofs. Note its date and vertical datum.
- Acquire a local copy (Operator: extract by extent) — the study area plus a margin, at a deliberately chosen resolution (see WCS), in the model’s own coordinate system.
- Make the surface hydrologically sound (Operator: fill sinks) — fill closed depressions so every cell can drain to an edge, keeping a minimal downhill gradient across filled areas.
- Compute slope (Operator: focal/neighbourhood) — the steepest gradient of each cell against its neighbours, in degrees (or percent).
- Compute flow direction (Operator: steepest descent, D8) — for each cell, the one of its eight neighbours that water flows to.
- Optionally, accumulate flow (Operator: flow accumulation) — how many cells drain through each cell; high values trace streams and drainage lines.
Output: a slope raster (degrees), a flow direction raster (eight directions) and, optionally, flow accumulation — on the same grid as the local terrain model.
Epistemic Choices to record
- Resolution: a finer grid resolves ditches, kerbs and small dikes that steer real water; a coarser grid gives smoother landscape-scale results. The two give different flow paths.
- To fill or not to fill: filling removes all depressions. In flat Danish terrain many depressions are real — lakes, wetlands and bluespots where water actually collects. Filling is right for tracing flow paths, wrong for mapping where water stands. Say which question you are answering.
- Bridges, culverts and tunnels: a terrain model shows a road embankment as a dam even where a culvert runs under it. Flow paths that stop at embankments may need burning in.
- Single (D8) or multiple flow direction: D8 sends all water to one neighbour; multiple-direction methods spread it. D8 is simpler to read and is what this Task produces.
- Minimum slope used when filling, and the units of slope (degrees or percent).
Task Recipes (tool-bound)
- QGIS: Slope and flow direction from DHM (Recipe - QGIS)
- Python: (not yet written — rasterio plus pysheds or richdem would implement the same Schema)