Detrending DEMs in QGIS or making REMs

Concept

Digital elevation models (DEMs) can be detrended into releative elevation models (REMs) when differenced against a trend surface. For example, removing the valley bottom slope or river slope along a riverscape is a common form of detrending. The HAND (height above nearest drainage) algorithm in TauDEM is an example of this. This is done based on 10 m NED elevation data for most of CONUS available here. But the algorithm can be run on high resolution topography. 

For example, this input DEM:

Can be detrended to just show elevations relative to the channel and water boides (or height above nearest drainage cell): 

Creating one yourself

There are lots of ways to potentially detrend DEMs in QGIS. 

• See this post for some ideas. 
• TauDEM can apparently be installed as a Plugin in QGIS (I have not tested this myself... we run it in the cloud)
• The Fluvial Corridor Toolbox is a QGIS plugin with the ability to detrend DEMs.

Creating a Polygon from a REM

Once you have a REM, you an threshold it to create an area above or below a certain value. We have a tool that does this for you in the QRiS plugin (the Raster Slider). The workflow is:

• Load your detrended surface to QRiS (e.g. directly loading HAND from a TauDEM project in Rivercapes Viewer). TIP: If you have an area of interest (AOI) you (like the red polygon below), you can clip the raster on import to QRiS.

• Once you have your HAND or REM surface in QRiS as context, you can right click on it and use the "Raster Slider" command. You can play with the threshold values and then generate a polygon.

See the process in this video: