The module r.stream.distance can calculate: the distance to streams; the distance to outlet; the relative elevation above streams; the relative elevation above the outlet. The distance and the elevation are calculated along watercourses.
The module may calculate the distance either downstream or upstream. The default is set on downstream. The upstream method calculates the distance to the local maximum or divide. Opposite to downstream method, where every cell has one and only one downstream cell, in upstream method every cell has usually more than one upstream cell. So it is impossible to determine interchangeable path from any cell.
The upstream method offers two alternative modes switched with -n flag: the nearest local maximum/divide is the shortest path to the local maximum; and the farthest maximum/divide is the longest path (default option). In hydrological sense, the nearest option means the shortest path which a particle of water must run from the divide to reach a particular pixel, while the farthest option means the possible longest path.
In outlets mode, the module can optionally be used for subbasins.
In streams mode (default) it calculates the distance (downstream) to the stream network taken in input. In outlets mode there are some additional possibilities. If the subbasin option is set off, it calculates the distance only for the outlet (downstream). If the subbasin option is set on, it calculates the distance to outlet for every subbasin separately. The subbasin option acts similarly to a subbasin mask. The module r.stream.basins can be used to prepare the stream network map taken in input by r.stream.distance. In fact it can be used to individuate basins and subbasins.
In lat-long locations, the module gives distances not in degrees but in meters.
In stream mode subbasin options is omitted. Input maps must be in CELL format (default output of r.watershed, r.stream.order and r.stream.extract). The distance is calculated in meters, for flat areas not corrected by topography. Distance correction by topography may be done using the following r.mapcalc formula:
r.mapcalc expression = "dist_corrected = sqrt(distance^2 + elevation^2)"
The module can work only if direction map, streams map and region have the same settings. This is checked by default. If resolutions differ, the module informs about it and stops. Region boundary and maps boundary may differ but it may lead to unexpected results. The elevation map is not affected by this restriction and can have whatever resolution.
It is also required that stream_rast and direction maps come from the same source, e.g. both from r.stream.extract. If the stream network was generated with MFD method also MFD direction map must be used.
Probably one of the most important features of r.stream.distance is the ability to calculate the distance not only for streams generated by r.stream.extract, but also for any integer map, as long as the resolution corresponds to that of direction map. It can be a lake, swamp, depression and lake boundaries even divided into smaller fragments each with its own category.
# Set the region to match with elevation map g.region -pa raster=elevation # Calculate flow direction and stream network r.watershed elevation=elevation threshold=10000 drainage=direction stream=streams # Calculate elevation above and distance to stream network using downstream method r.stream.distance stream_rast=streams direction=direction elevation=elevation \ method=downstream distance=distance_stream_downstream difference=difference_stream_downstream # Calculate elevation above and distance to stream network using upstream method r.stream.distance stream_rast=streams direction=direction elevation=elevation \ method=upstream distance=distance_stream_upstream difference=difference_stream_upstream # Create outlet echo "636645,218835" | v.in.ascii -n input=- output=outlet separator="," # Convert outlet to raster v.to.rast input=outlet output=outlet use=cat # Calculate distance to and elevation above outlet r.stream.distance -o stream_rast=outlet direction=direction elevation=elevation \ method=downstream distance=distance2outlet difference=difference2outlet
See also r.streams.* modules wiki page.
© 2003-2020 GRASS Development Team, GRASS GIS 7.8.3dev Reference Manual