Why is my flow length tool giving value less than one? Isn't a flow length tool supposed to give the maximum length of upstream flow to the cell? Then, how come?

I am using coordinate system of GCS WGS 1984. Is this happening because my coordinate system is in degrees? How can I rectify this problem?

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2 Answers 2


You already figured out your problem. The unit of measure is in degrees, therefore your raster is a measure of flow length in degrees.

Before doing your analysis, you need to use the Project tool to re-project your data into a projected coordinate system so that it uses a linear unit. If you have not worked with projections much, be sure to use the Project tool and NOT the Define Projection tool.

  • I reprojected the data into WGS 1984 UTM zone 43N which has linear units. But now the problem I am having is that it is taking like forever for the flow length tool to run (even for half hour it didnt complete). Can you give me an alternate way to get my flow length? Like multiplying my above generated raster by something or something like that? Sep 29, 2016 at 14:48
  • Raster operations like this can run a pretty long time. I would say it is not uncommon for this kind of raster operation to take much greater than 30 minutes for the geographic extent that it looks like you're using. Because geographic and projected coordinate systems are inherently measuring different things (flat surface vs. curved surface), just converting the raster you have in degrees would likely be relatively inaccurate. Sucks that it takes such a long time, but my suggestion would be to set the tool up before you go to lunch or something and let it run while you're away.
    – mmoore
    Sep 29, 2016 at 15:19

You need to Project (or batch project if shapefiles) to Projected Coordinate System (coordinates come with units like metres, cm, etc) using Data management Toolbox in ArcToolbox. This should solve the problem.

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