I need help with qgis flow path length.
I try to run the program and when I run the program I get a raster with 0 values, as input I'm using filled raster, I don't now if I'm doing something wrong or not but i can´t get any map.
[GIS] Determining flow path length in QGIS
flowqgissaga
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It seems that all the values in slopes are rounded to the nearest 0,5. So 0,401 m / 2.8284 m = 0,14177, would instead be 0,5/3 = 16,6667. Or if distance is 2 metres, then 0,5/2 = 25 %. In the manual it says the values are rounded in FLAT areas, which seems to be a little off considering it does that where slope is 25 %. Another remark is that in flat areas (lakes) the drop percentages are indeed continuous!
It is very difficult to know exactly where things went wrong without knowing details about the inputs that you used for each tool. However looking at the two images that you embedded into your post, it looks to me like the common problem of running a flow path analysis on a non-hydrologically preprocessed DEM, i.e. a DEM for which you have removed all of the topographic depressions and flat areas through either depression filling or breaching. The ArcGIS tool requires an input of a D8 flow pointer raster, which clearly you provided from a depressionless DEM. I think you may have provided the r.flow tool input the original DEM which contained depressions. Take the following images as examples:
The first image is a maximum upslope flow-length grid calculated from a depressionless (breached) DEM and the second one was derived from the original DEM, which contained numerous artifact topographic depressions. You'll notice that the flow paths have been severely truncated because as flow encounters a depression or flat area, there are no downslope grid cells for the flow path to continue towards. Just as is the case with your data, the maximum flow path value in the grid is considerably less in the second raster as a result of this flow path truncation. I think you need to double-check that you input a hydrologically corrected DEM into the r.flow tool. I know from experience that this is one of the most common errors when performing DEM-based hydrological analysis. Otherwise, it is possible that you simply provided the r.flow tool the wrong input, e.g. the same D8 flow pointer that you provided the ArcGIS flow length tool (note r.flow requires a DEM and not a flow pointer as an input). Again this would be a fairly common mistake and the effect would be identical, i.e. truncated and discontinuous flow paths which is what you seem to have. I hope that helps.
Best Answer
You should check your DEM or try to update your QGIS. I just tried with QGIS 2.18.16 and It worked.