I am interested in producing a isochrone map given a raster speed map and a direction map (like the output of
r.watershed), but I do not see how to produce it.
I have studied
r.cost wich calculate accumulated cost on a friction map, but it is not intended to work with a direction map.
I have been trying to see if there is a way to produce the inverse of an accumulative map like the result from
r.drain but backword, so given the outlet and the direction maps, assign a cumulative cost for each cell of the catchment area from the sarting point (downstream).
I am familiar with GRASS and QGIS, but any solution with a free software is welcome.