I am interested to know why ‘Flow Direction Raster’, derived from ‘Filled DEM’, generates values ranges from 1-128, with pixel depth of 8 bit unsigned integer which is readily capable to store values 0-255, As such, in the Tool’s Help, it is clearly stated that “the output of the Flow Direction tool is an integer raster whose values range from 1 to 255”. My ‘filled DEM’ carries 16bit signed integer pixel depth already, which has capacity to store values range from -32768 to 32767!
Answer
There are eight valid output directions relating to the eight adjacent cells into which flow could travel. This approach is commonly referred to as an eight-direction (D8) flow model, the direction coding is shown:
If the water will flow to the cell immediately to the right, it will be assigned a code of 1, if it flows to the top right cell then the code assigned is 128 as so on.
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