I am trying to replicate a flow length calculation in arcmap 10.2. I have an elevation raster with a pixel range from 0 to 3038.96m (32 bit). When I run the flow direction calculation, the pixel range drops to 0-255 (8 bit). I must be losing information that will affect my flow length calculation (I have a flow length calculation that has been completed that I'm trying to match)--how do I prevent this? I've tried searching in environment settings but I haven't found how to change my pixel range.

I'm fairly new to GIS.

2 Answers 2


The flow direction is indeed an 8 bit image, irrespective of your input (32 or 64 bit float), from the Esri help page:

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From the centre pixel (blue) the value indicates which way the flow will go from 1 (East) to 128 (Northeast)..

From flow direction you need to proceed to Flow Accumulation or Flow Length which uses this direction raster to calculate the flow from each cell into the next; as this cell flows into another then that cell flows into the next one... from that accumulation and length can be gained.


The very first step is Fill. It fills sinks in elevation model. Next - flow direction on filled DEM. After that you can do all other things, like flow length, using flow direction.

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