DEM derivatives - get more than you bargain for!

DEM derivatives: Get more than you bargained for

As touched on in last week’s blog, one of the main advantages offered by digital elevation rasters, or digital elevation models (DEMs), is that they can be used to derive a whole suite of additional data products. These products are quick and easy to generate, and can provide invaluable insight into the terrain characteristics of an area.

Let us take for example the following region in the Fetakgomo municipality:

From the aerial photograph we can gather that the area contains a small village and is quite arid. The region appears to exhibit some topography, but it is not explicit.  However, the terrain of the region becomes much clearer when the DEMSA2 is visualized.


The figure above is known as a shaded relief map (or hypsometric tinting). It is essentially coloured elevation values, overlayed onto a hillshade of the area. The hillshade provides morphological ‘texture’ and highlights even small surface features, such as buildings, that are too small to result in a significant colour (elevation) variation.


The hillshade itself (figure above) is the first DEMSA2 derivative that we will be considering. It is created by illuminating the DEM with an artificial light source, and then capturing the resulting shaded areas created by the surface model. Hillshades are generally used for quick visual inspections of topography, due to it being easy to interpret and requiring less memory than a full elevation model to display (because the hillshade grey values range from 0 to 255, whereas the elevations can range from 0 to 8848m).


Next is the slope gradient derivative (above), which represents the change in elevation. Slope gradient is extremely useful for a wide range of geospatial applications. One example would be site-selection, as it can be used to easily filter out (exclude) regions of steep slope.  It is also very useful for land cover classification applications - note how clearly visible the buildings in the village are in the figure above.


Aspect (above) is another derivative that can be generated from the DEMSA2. Simply put - aspect indicates the direction a surface (slope) is facing. The figure above shows the aspect (given in degrees, but classified into four cardinal compass directions) of the area. By itself, aspect may not seem very useful, but it can be used in conjunction with other derivatives (such as slope) to generate a range of other datasets, such as annual solar radiation for the region (below).



Solar radiation is particularly useful for solar power plant scoping. At 2m resolution, it can even be used to identify areas on building roofs that receive the most solar radiation.

The final derivative we will be considering in this post is the topographic wetness index (TWI). This dataset (shown below) provides an indication the flow and accumulation of water in a landscape. We use the TWI in our flood hazard index (a topic for a next post), but it can also be used for other hydrological applications such as identifying areas within agricultural fields where water may accumulate (i.e. where drainage need to be improved).



The above DEMSA2 derivatives are only a subset of what we include in our Geodatapacks. There are also a range of other derivatives that can be created from our elevation data.

Published on 2020-10-15.

 

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