Terrain & Hydrology

Purpose: terrain products whose numbers are physically meaningful. The two recurring failure modes: unit mismatch (degree coordinates with meter elevations silently corrupts every derivative) and unconditioned DEMs (flow routed into spurious pits produces fragmented, fictional streams).

DEM hygiene first

Check	Rule
Surface type	DTM (bare earth) for hydrology/slope; DSM (with canopy/buildings) for viewshed/solar. Using a DSM for watersheds routes rivers over treetops.
Source	Copernicus GLO-30 > SRTM for most global work; national LiDAR DTMs when available (see point-cloud-lidar to make your own). Record source + acquisition date.
Nodata	Identify the nodata value (-9999, -32768, 3.4e38) and mask it — never let it enter statistics or fill algorithms as "very deep hole".
Voids	Fill data voids (interpolation from edges) BEFORE hydrological conditioning; document filled areas.
CRS + units	Reproject to a projected CRS so horizontal units = vertical units (meters). Slope from a 4326 DEM without z-factor correction is the classic silent error. If staying geographic, apply a latitude-dependent z-factor — better: don't.

Derivatives

import whitebox
wbt = whitebox.WhiteboxTools()
wbt.slope("dem.tif", "slope_deg.tif", units="degrees")
wbt.aspect("dem.tif", "aspect_deg.tif")
wbt.plan_curvature("dem.tif", "plan_curv.tif")

• Slope: state units (degrees vs percent — 45° = 100%); Horn's method (3×3) is the standard; steeper terrain → consider resolution effects (slope flattens as cell size grows — report cell size with every slope statistic).
• Aspect: circular variable — never average it arithmetically; use vector (sin/cos) averaging; flat cells have undefined aspect (mask, don't zero).
• Curvature: plan (flow convergence) vs profile (flow acceleration) — pick per question.
• Hillshade is for cartography (see cartography-geoviz), never analysis input.
• Ruggedness/position: TRI, TPI (radius-dependent — report the radius), geomorphons for landform classification.

Hydrological conditioning — order matters

voids filled → breach depressions (preferred) → fill remaining pits
→ flow direction → flow accumulation → streams → watersheds

• Breaching before filling (WhiteboxTools BreachDepressionsLeastCost): carves through barriers (road embankments over culverts) instead of flooding upstream areas flat. Pure fill on flat/embanked terrain creates large artificial lakes with arbitrary flow paths.
• Real depressions exist (karst, prairie potholes, reservoirs). If the landscape genuinely holds water, don't condition it away — model with explicit sink handling and say so.
• Flow direction: D8 for stream networks/watersheds (discrete, standard); D-infinity/MFD for dispersal quantities (wetness index, erosion) on hillslopes.

Streams and watersheds

• Stream extraction threshold (min. accumulation) is a MODELING choice: derive from a mapped reference network (match total stream length) or report the threshold and show two alternatives — never present one threshold's network as "the" rivers.
• Pour point snapping: outlet coordinates rarely fall on the modeled stream cell. Snap to the highest-accumulation cell within a search radius (wbt.jenson_snap_pour_points) — an unsnapped pour point yields a tiny, wrong watershed silently.
• Verify delineation: watershed area vs authoritative basin data (±5-10%), and the modeled network overlaid on imagery/topo maps at 3 locations.
• Wetness index (TWI), stream power (SPA): compute from conditioned DEM + MFD accumulation; they are relative indices — don't read absolute thresholds across regions.

Viewshed

• Use a DSM (or DTM + feature heights) — bare-earth viewsheds overstate visibility wherever trees/buildings exist; state which surface was used.
• Set observer height (~1.7 m person, tower height for infrastructure) and target height explicitly; defaults differ across tools.
• Account for earth curvature + refraction beyond ~5 km (wbt.viewshed handles it; verify the flag).
• Deliver binary visible/not plus the observer point(s) and parameters in the metadata; for siting problems, cumulative viewsheds from candidate sets feed mcda-suitability-analysis.

Tooling

WhiteboxTools (conditioning, full hydrology suite, fast) · pysheds (lightweight Python watersheds) · richdem (derivatives) · GDAL (gdaldem) for quick slope/hillshade · GRASS (r.watershed) for very large DEMs (no explicit fill needed — least-cost routing).

Verification protocol

1. Derivative histograms: slope > 60° over large areas or negative accumulation = unit/nodata bug.
2. Stream network overlay on imagery at 3 locations, including one flat area (where artifacts concentrate).
3. Watershed area cross-check vs authoritative basin polygons.
4. Report: DEM source/date/resolution, conditioning method, flow algorithm, stream threshold, all in the deliverable.

Pitfalls checklist

• Slope from a geographic-CRS DEM without z-factor (values ~100× off).
• DSM used for watershed delineation (rivers over treetops).
• Fill-only conditioning across road embankments → phantom lakes.
• Unsnapped pour point → 3-cell "watershed".
• Arithmetic mean of aspect (350° and 10° average to south, not north).
• Nodata treated as elevation in fill/statistics.
• One arbitrary stream threshold presented as the drainage network.

Execution contract

• Workflow: inspect DEM source, CRS, vertical units, datum, resolution, and nodata; condition terrain; derive gradients and flow; delineate products; test thresholds; validate against imagery and controls.
• Decision rules: use terrain workflows on raster elevation products, point-cloud workflows before DEM generation, and choose conditioning and flow algorithms from landscape and scale.
• Verification protocol: inspect derivative distributions, hillshade artifacts, stream overlays, watershed area, pour-point snapping, threshold sensitivity, and elevation-control residuals.
• Failure modes: reject products from DSM misuse, geographic-unit slope, vertical datum mismatch, unconditioned barriers, nodata contamination, unsnapped outlets, or resolution unsupported by source data.
• Deliverables: conditioned DEM, derivatives and hydrologic products, parameter and threshold record, CRS and vertical datum, QA maps, validation metrics, and limitations.
• Source freshness: consult the authoritative source registry before applying tool algorithms or product rules and record the checked date.