From elevation to flow direction
Begin with a terrain surface whose cells share a vertical datum. Inspect sinks before applying a fill operation: real wetlands and quarry voids should not disappear merely because an algorithm expects continuous drainage.
Contour intervals and scale
The reference drawing uses a nominal 10 m interval, with heavier index contours every 50 m. Intervals are a portrayal choice. They do not imply a 10 m vertical accuracy or resolve small banks between sampled cells.
Rivers across tile edges
Keep a stable reach identifier through clipping. A channel should meet its neighbouring tile at the same coordinate and retain its downstream orientation. Inspect edge continuity before simplifying bends for a small-scale overview.
Uncertainty at the divide
Broad plateaus, engineered drainage and very flat floodplains need manual interpretation. Publish an uncertainty corridor where plausible drainage choices differ, rather than drawing a precise line unsupported by the terrain.