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
Answer
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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