Charteris Bay & Mt Herbert
Real LINZ 8 m DEM · 500 × 500 cells · every step computed live
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1Condition the DEM
2Extract streams
3Delineate a catchment
Cell inspector (D8)
Guided Questions
Switch to Flow accumulation. Hover over a ridge crest, then a valley floor. What values do you get? Now look at Flow direction in the inspector: which neighbour does D8 pick, and why that one?
Turn on the OpenStreetMap streams. Find the threshold where the extracted network best matches them. Write down the threshold and the network length. Where does the DEM put a stream that the map doesn't have, and which one is wrong?
Untick Fill sinks and note the number of sinks. Now switch the precision to 0.1 m, then 1 m. It is the same terrain, so where did the extra sinks come from? Place a pour point near the coast on the largest stream and compare its area with filling on and off. Are most sinks real depressions or DEM artefacts?
Drop a pour point on a stream. Then untick Snap and click one or two cells beside the channel. Why does the catchment collapse? How does this explain a delineated catchment boundary that runs straight through a gully?
Imagine a coastal road crossing the main stream. Delineate its catchment and read the rational-method peak flow. Move the culvert 500 m upstream: how do the area and Q change? Which input (C, i or A) would you trust least, and why?