I'm trying to generate rasters whose values summarize the mean value of a given parameter (slope, curvature, etc.) of the upslope terrain. I feel like this should be doable using only a flow direction raster and a raster of the parameter of interest, but I don't really know where to begin. I've seen this instruction http://forums.esri.com/Thread.asp?c=93&f=995&t=303114, but I can't seem to figure out a way to interpret it that produces a result that makes any sense. I'm familiar with ArcMAP, QGIS, SAGA, and a bit of GDAL and GRASS, so I'll gladly accept instructions for any of these platforms.

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    Jay, Could you elaborate on your problem with my prescription on the ESRI forums? It merely amounts to summing the upslope values and dividing by their counts, which is the very definition of the (arithmetic) mean. Details of the FlowAccumulation operation in Spatial Analyst are at help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//…. – whuber Oct 26 '12 at 21:47

Thanks for the reply. I actually figured it out yesterday and hadn't had a chance to mark this as "solved." I had been interpreting the advice on the ESRI forums as the following steps:

  1. Create a slope raster, then run the flow accumulation tool on it.
  2. Create a raster with a constant value, then run the flow accumulation tool on it.
  3. Divide result from step 1 by result from step 2.

This didn't make very much sense to me, but I tried out a couple variations anyway, with interesting but nonsensical results. Eventually, I realized that what I was supposed to be doing was running flow accumulation weighted by the slope values (sum of slope uphill from a cell), then dividing this by the unweighted accumulation (total area uphill from the same cell). So, even though I initially misinterpreted this advice, it did get me looking in the right direction.

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