Relative Elevation Model

Elevation expressed relative to the water surface of a nearby river, rather than relative to sea level. Produced by detrending a bare-earth DTM: sample DTM heights along the river centreline, interpolate a smooth trend surface across the valley, then subtract it from the DTM. The result flattens the regional slope so that abandoned channels, meander scrolls, terraces, and floodplain micro-topography stand out, and it colour-maps directly to height-above-river for flood context. Distinct from HAND (height above nearest drainage), which measures vertical distance to the drainage network along flow paths. A REM is a derived product, not a measured one: it is one detrend transform away from a DTM (which is itself one classify-and-grid transform away from a lidar point cloud). Folia has no detrend op yet — this concept names the output that recipe should produce. Common tools: RiverREM (OpenTopography), WhiteboxTools, manual interpolation in QGIS.