I'm trying to use ArcGIS Desktop to delineate watersheds for a series of sampling sites. I've been following the protocol laid out by WhiteboxDev in General steps to create watershed boundary in ArcGIS Spatial Analyst?

However, I'm having troubles with the flow accumulation tool. When I run it based on my flow direction raster, I end up with values that range from zero to nine million, which seems wrong, and can't be used for the next steps.

Screenshot of flow accumulation tool result.

Does anyone have an idea why this might be happening, and what I can do to fix it?

I haven't used any other layers to weight the output, and I think my flow direction layer is accurate.

Screenshot of flow direction layer.

I'm using the NASA SRTM elevation data, which has 1 arc-second (~30 m) cells. In Arc, the cell dimensions are listed as (0.00027777778, 0.00027777778), which I assume is in degrees. I understand the idea of the flow accumulation tool, I just don't understand why it's skewing all of my data.

  • 3
    The flow accumulation values are the total count of uphill cells that flow into that cell. What are the dimensions of your input raster- the x and y counts? If you are working with a high-resolution LiDAR dataset, say with 3ft pixels, that 9 million count watershed only needs to be 9000ft by 9000ft. – GBG Dec 2 '16 at 21:29
  • Have you tried projecting your data to a projected coordinate system? – Aaron Mar 27 at 1:52

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