Limitations

The technology used to generate watersheds has some limitations:

• To determine the catchment for a given site, we use Digital Elevation Models. These data represent the surface of the earth and can only be used to determine the water flow above the ground. Underground waterways (such as sewers) are only detected by the system if the sewer network is available and has been correctly digitized.

• The precision of the data used can be a limitation. The default DEMs available (SRTM30m, ALOS30m, FABDEM) have a precision of 30m. This means we calculate the average elevation for a grid square of 30mx30m. Consequently, small waterways are harder to detect with such precision. Similar limitations apply to population data. Typical population rasters have a precision of 100m, meaning that the number of people is estimated for a grid square of 100mx100m. It should be noted that the population data are estimates and they have limitations themselves as well. Please refer to the release notes of the corresponding population model to understand its limitations.

• DEM data represent the elevation at the surface of the Earth. These elevation data do not always eliminate noise (such as buildings, or trees). In some cases, the elevation data of a bridge over a river could even correspond to the elevation of the bridge instead of the river. In this case, the tool would detect a discontinuity in the river and could thus give inaccurate results. Digital Surface Models (DSM) do not distinguish trees, buildings and such. Digital Terrain Models (DTM) represent a more accurate version for our purposes.