Analyze and simplify the topology of massive raster terrain models with SCALGO Topology

Illustration of how a volume depression mapping (volume-bluespot) gives a simple flooding risk map where cells with a small volume (red) have high flood risk and cells with a large volume (green) have low flood risk.

The term bluespot is often used for cells that are in a major depression, that is, the depressions that are not contained within other depressions. These cells are exactly the cells given a value by the Depression Mapping module. Bluespots are often used to give a rough estimate of areas that could be vulnerable to flooding during extreme rain (where water gathers in depressions). As illustrated below, the Depression Maping module can be used to give a much more detailed measure of the vulnerability of each cell in a depression, since it e.g. computes for each cell in a depression the volume of water required before the cell becomes flooded by water rising in the depression. The SCALGO Flash Flood mapping product goes a step further and determines the vulnerability based on both the volume and the size of the upstream area.

The Depression Identification and Depression Filling modules are part of a two step process for topologically simplifying a terrain model, where Depression Identification computes terrain topology and Depression Filling then performs the simplification. The modules can be invoked separately to avoid recomputing topology when simplifying the same terrain models with different simplification thresholds. The statistical information output from the Depression Identification module can e.g. be used to determine what threshold to choose in order to obtain a certain simplification (a terrain with a specific number of depressions).

Modern detailed (and thus massive) terrain models often contain a large number of depressions that may adversely affect terrain analysis applications (such as surface flow modeling) and derived products (such as bluespots or contour lines). This leads to the need for terrain model simplification, but often such simplification is done in a very coarse or ill-specified (heuristic) way that may lead to the loss of important features and thus problems in subsequent applications. In contrast, the Depression Filling module simplifies a terrain model in a fully-specified way. This leads to significantly improved derived products and analysis results.

For example, when modeling how water flows on the surface of a raster terrain model, stream networks are often extracted as the cells that transport a lot of water (blue on the below figures). In this case, depressions in the terrain will impede flow leading to a disconnected stream network (left figure). Therefore depressions are often filled before modeling flow to simulate a situation where no surface water gathers in depressions (middle figure). However, filling all depressions can lead to loss of a lot of detail (and very large flat areas) and therefore unrealistic stream networks; filling all depressions in the above-right terrain example would remove most of the terrain detail. The Depression Filling module can be used to only remove (fill) "insignificant" depressions, leading to more realistic stream networks while maintaining most of the terrain detail (right figure).

The output of the Flat area module also marks the size of the flat areas.

The output from the Flat Areas module is a raster where each cell corresponding to a flat area cell in the input raster contains the size of the flat area (and nodata otherwise). This output can for example easily be used in combination with the Calculate module in the SCALGO Utility software package to remove flat areas above a certain size x, simply by setting all cells with a larger value than x to nodata.