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.walk and 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.

  • CellSize/Velocity is time required to cross 1 cell. Use it as weight raster in flow accumulation.
    – FelixIP
    Commented Jun 23, 2020 at 23:00
  • The weighted flow accumulation will give the higest value for the last point (outlet) and I want the opposite, the lowest value for the lowest point and the highest value for the farest or highest point
    – Marco
    Commented Jun 24, 2020 at 6:21
  • 1
    Isochrone Variable Speed from SAGA (tool not implemented in QGIS) should help you doing what you want ! You could also have a look at r.traveltime in GRASS but in my opinion the tool from SAGA is more powerful
    – wanderzen
    Commented Jun 24, 2020 at 8:20
  • Ok. Use it as weight raster in flow length.
    – FelixIP
    Commented Jun 24, 2020 at 8:50
  • @FelixIP, flow length? where is that? That sounds promesing. Does it assing a lenght to a given output to all cells?
    – Marco
    Commented Jun 24, 2020 at 9:55

2 Answers 2


Here how "Isochrone Variable Speed" from SAGA works:

  1. First you need to load your filled DTM into SAGA
  2. Then you need to run the "Slope, Aspect, Curvature" tool as shown in the two pictures below. (only the slope layer will be used in the "Isochrone" tool)



  1. Then click on "Geoprocessing" ; "Find and run tool" and look for "catchment"

  2. Select the catchment area method you want to use

  3. Select in "grid system" the parameters associated to your DTM and use your DTM in the "elevation grid" line

  4. In "method" use "Needs more memory but is quicker" (this step might be very long, if it is the case I would advice you to add directly a catchment area layer calculated in QGIS)

  5. Once your catchment area layer has been added to your saga project (as shown below), double click on it to add it to the map.

  6. Then go in "Geoprocessing" "Terrain Analysis", "Hydrology", "Dynamics", "Isochrone Variable Speed"

  7. Once you have set up the inputs, click on the black arrow as shown below:


  1. Once the arrow is selected you can directly click on your outlet to run the "isochrone variable speed" tool (if your drainage basin is big it might take a few minutes for the results to be loaded)

You should get something like that:

Isochrone output (in hours):

Isochrone output (in hours):

SAGA GIS is not as intuitive as QGIS to use but it doesn't take too long to get familiar with ! Let me know if you have some troubles running the tool ! :)

MAJ QGIS 3.22 and < :

The Isochrone Variable Speed tool is now available in QGIS within the SAGA core plugin

  • I got lost quite soon in SAGA, I need more time to figure out what the work flow is in general in SAGA. I loaded the raster as a grid and then in the "interactive" part I did not know how to proceed
    – Marco
    Commented Jun 25, 2020 at 8:12
  • 1
    I've edited my answer so you can see how the tool works :)
    – wanderzen
    Commented Jun 25, 2020 at 10:55
  • Great! I will try it again later. The black arrow!!! I would have never figured it out myself!
    – Marco
    Commented Jun 25, 2020 at 12:51
  • I owe you the bounty. I was away and it expired without an action from me. :(
    – Marco
    Commented Jul 5, 2020 at 6:52

There's another trick you might try: You can invert the DEM, so that ridges become valleys and stream outlets become the peaks. Then run r.watershed, and the resulting flow accum should be highest at the original "upstream" mountains, and lowest at the original drainage points.

  • Thanks. I remember I have read about it somewhere here, but when I looked for it I could not find it. I need to try it, so I will not forget it
    – Marco
    Commented Jul 5, 2020 at 6:57

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