terrain-hydrology

# 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 ```python 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%); ...