A flood-resilience team is screening boroughs for a tidal-surge study and wants a first-pass exposure figure from the Copernicus DEM: how much of each borough's area lies below ten metres above the geoid. It is a screen, not a model — the DEM is a surface model and the Thames Barrier exists — but the figure decides which boroughs get the detailed study, so it has to be an area, in square kilometres, not a pixel count.
Return one feature per borough — the borough polygon — carrying:
namelow_km2 — the area of the borough with elevation below 10 m, km², three decimalslow_pct — that as a share of the borough's DEM-covered area, percent, two decimalsThe DEM grid is geographic: a pixel is about 19 m east–west and 31 m north–south here, and its area varies with latitude. Compute the area honestly — reproject the DEM to British National Grid before counting, or weight each pixel by its own ground area. Nodata is −32767 and there is none inside the clip. Return all 33, in EPSG:4326, most exposed first.
These are where the teaching is. Read them twice.
Area-below-threshold from a DEM is the first screen in coastal and fluvial flood studies, and the pixel-area mistake is the one reviewers look for on any geographic-grid product.
6 scored, 0 informational
One feature per borough
17%Carries name, low_km2 and low_pct
17%Total low ground across all boroughs
33%The most exposed borough's share
17%Result is polygons
8%Submitted in EPSG:4326
8%