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The range of values in my flow direction raster is 1-129.

D8 method was used.

I would expect 128 to be the largest value in accordance with the documentation

what's more I show values OTHER THAN 1, 2, 4, 8, 16, 32, 64, 128

Why are there these other values?

One answer to this question begins to suggest this is normal but does not really explain why/how other values could be produced. Flow Direction Raster Value Range

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    Start with fill, to make tools work. – FelixIP Mar 28 at 20:33
  • I'm trying to selectively fill areas so accumulate flow from multiple near by flow parth to a single pour point so fill will kil my work gis.stackexchange.com/questions/317075/… – Zipper1365 Mar 28 at 20:38
  • If you are getting values other than 1, 2, 4, 8, 16, 32, 64, 128 then your input DEM has not been passed through the FILL tool to remove sinks. The flow direction will not generate a sensible raster surface if the input data has sinks, this then has knock on effect on flow accumulation and watershed tools. So as @FelixIP says run it through the FILL tool first. – Hornbydd Mar 28 at 22:12
  • Fair enough. it sounds like Like I'm asking it to do something it's not built to do. I have selectively left areas unfilled to force flow into a valid sink and want all local flow to converge on those single pixel sinks. – Zipper1365 Mar 29 at 13:12
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According to the ArcGIS Help file:

  • If a cell has the same change in z-value in multiple directions and that cell is part of a sink, the flow direction is referred to as undefined. In such cases, the value for that cell in the output flow direction raster will be the sum of those directions. For example, if the change in z-value is the same both to the right (flow direction = 1) and down (flow direction = 4), the flow direction for that cell is 1 + 4 = 5. Cells with undefined flow direction can be flagged as sinks using the Sink tool.

  • If a cell has the same change in z-value in multiple directions and is not part of a sink, the flow direction is assigned with a lookup table defining the most likely direction. See Greenlee (1987).

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