Creating REM from DEM in QGIS

 REMs are best produced from LiDAR data, however my region of interest has no publicly available LiDAR dataset. So, I will use high-resolution DEM from "Copernicus Global Digital Elevation Models" distributed by OpenTopography because it offers superior horizontal and vertical accuracy compared to SRTM DEM data. To cite this data, use:-

European Space Agency (2024). Copernicus Global Digital Elevation Model. Distributed by OpenTopography. https://doi.org/10.5069/G9028PQB. Accessed 2026-09-05

This study area is the Uke river (approximately Latitude 8°35'N to 9°05'N and Longitude 7°35'E to 7°55'E) that streches from Karu LGA into Nasarawa LGA all within Nasarawa state. The Uke river is a dynamic, seasonal river system that feeds into the larger Uke-Antau-Mada drainage basin, just east of Abuja.

What is Relative Elevation Model

A Relative Elevation Model (REM), also called a detrended digital elevation model or height-above-river raster, is a specialized 3D map that shows land height relative to a nearby river channel rather than above sea level.

Traditional Digital Elevation Models show absolute height above sea level. Because rivers naturally slope downward from their source to their mouth, this overall slope can hide small, local changes in the landscape.

An REM "flattens" the river's downward slope by setting the riverbed elevation to zero everywhere.

Software takes a standard elevation grid, estimates the water level along the river's center line, and subtracts that predicted river height from the original elevation data.

A value of zero means the ground is level with the nearest river channel, while positive numbers show how high a specific spot sits above that river.

Steps for Creating a Relative Elevation Model

Step A: Reproject and Clip

• Open your downloaded GeoTIFF in QGIS.
• Reproject the layer from WGS 84 (Geographic) to a projected coordinate system for accurate distance measurements. For Nasarawa State, use WGS 84 / UTM Zone 32N (EPSG:32632).

Step B: Extract River Vector and Elevation

1. Manually digitize the centerline of the Uke River as a line vector layer, or extract it automatically using the SAGA Channel Network tool.
2. Use the Extract Vertices tool to convert the Uke River line into a series of dense point features along the channel.
3. Open the Sample Raster Values tool. Use your reprojected DEM to add the exact elevation (Z-value) to every point along the river channel.

Step C: Create the Arbitrary River Plane

1. Open the Inverse Distance Weighting (IDW) Interpolation tool under the QGIS Processing Toolbox.
2. Set the input layer as your sampled river points and select the sampled elevation attribute as your vector interpolation field.
3. Set the extent to match your clipped study area and run the tool. This creates a smooth, continuous "water level baseline surface raster" that mimics the downhill slope of the river.

Step D: Calculate the Relative Elevation Model

1. Open the Raster Calculator.
2. Enter the formula: [Projected_DEM] - [IDW_Interpolation_Surface]
3. Run the calculation.

The resulting layer is your REM. All values of 0 will represent the exact channel bed of the Uke River, while higher values represent the exact height of the surrounding terrain relative to the closest point of the riverbed, perfectly highlighting paleochannels, floodplains, and terrace heights.

Thank you