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I am trying to calculate the slope change in ArcGIS from a DEM. Thus far I have calculated the slope in degrees and calculate the slope in percent from that. (see: Calculating changes in slope angle between cells/areas using ArcGIS Desktop?)

As ArcGIS calculates absolute values I cannot detect if the slope increases or decreases. For my further analysis I want only the increasing slopes (e.g. change > 30%).

Is this possible in ArcGIS?

closed as unclear what you're asking by PolyGeo May 14 '17 at 3:24

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  • it seems that you need another dataset to complete this analysis. The only thing you can identify from the DEM with the slope tools is the slope of the terrain. It sounds like you are modelling erosion scenarios. You should look at the hydrology tools within the spatial analyst toolbox. They might provide the tools you need for this process. I think you are on the wrong track with slope. Hydrology tools allow you to identify catchment basins, flowpaths, accumulation points, etc. – jbalk Jan 25 '17 at 3:12
  • Similar to @jbalk's comment, I think the part you're missing is that you need to tell the GIS from where you want to know uphill/downhill. For example, a hillside is both uphill (from a valley) and downhill (from the top of the hill). I'd guess flow accumulation is what you're looking for, which tells you which pixels are more downslope than others. – phloem Jan 25 '17 at 6:04
  • do you mean concave vs convex slopes ? – radouxju Jan 26 '17 at 10:10
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You can select by attributes the slopes that are greater than 30%.

Or if you're looking for slopes in a particular direction you will have to take further steps. The only difference between an "increasing" or "decreasing" slope is the direction the slope faces, if I'm understanding your question.

  • I think I understand what you are asking. You want +30, not -30, but ArcMap only returns |30|, which this answer doesn't address. – MaryBeth Jan 24 '17 at 15:14
  • I might have to rephrase my question: I want to find out if there is an ascend or descend of the landscape via the slope. My final slope delivers values from 0-~140. But all these slope changes could be either downhill or uphill changes - whereas I only want the uphill changes. – Carlos Jan 24 '17 at 15:15
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    How are you determining uphill/downhill? Is there a starting point, then radiating outward, or is there a path that is being followed? – Bjorn Jan 24 '17 at 15:40
  • I think my english might not be sufficient enough to explain the problem. My basics steps are as given in the link (last answer). But the received values represent both negative and positive change in slope; yet expressed as absolute values. I want the raster cells where eroded material can get accumulated which would be the cells where the landscape rises again. – Carlos Jan 24 '17 at 16:05
  • Whenever there's a slope there's a high end and low end. The direction of travel will determine if it's ascending or descending. I think in your case every raster cell with a slope value would be ascending in some manner. – AMgis Jan 26 '17 at 4:04

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