1

I created a cost map from an elevation map an a (degree) slope map using r.walk (version released on 2015-03-12) in QGIS 2.14.3. According to the documentation the resulting map should display the time needed to reach each raster cell from my starting point. As far as I understood, the unit of time should be seconds. I wanted to mask the area reachable in 3600 seconds, but it is only about 800 m wide on more or less flat terrain.

Could the unit of time be something else than seconds? Or is there some other error in my thinking.

  • It is seconds. You have to provide more information about the inputs. What is your friction for example (use r.univar) and the other parameters. – Anna Jun 19 '17 at 14:04
  • @Anna Inputs where a ASTER DEM with approximately 25 m resolution. I derived a slope map with r.slope.aspect with default settings. I run r.walk with default settings as well. – akra-kachi Jun 19 '17 at 15:02
  • You don't need slope map. You provide elevation raster and friction raster. Friction raster can be raster map of zero for start (such as r.mapcalc "friction = 0"), please read the manual page to understand the inputs. – Anna Jun 20 '17 at 1:27
  • You can actually use a slope map as an approximation of friction, if you don't have anything better. However the output depends heavily on the lambda-value (default 1.0). For my data a very small lambda-value (0.02) produced results, that where (based on own walking experience in the area), quite realistic. – akra-kachi Jun 23 '17 at 9:34
  • The slope along the path is computed within the module from the provided elevation. It is possible to provide slope map as friction, but that is steepest slope, and I think it makes sense only in very specific cases. – Anna Jun 24 '17 at 11:54

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.