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Using TauDEM I've created a number of derived raster products from an initial 30m SRTM DEM with a statewide (South Carolina) extent. Using the D8 modelling methods, I've created both the D8 Flow Direction and the D8 Flow Accumulation secondary inputs. Next, using the Gage Watershed method, I've rendered the catchment above a pour point. However, I'm struggling to create an output of the longest watercourse in the basin.

I have executed the Grid Network method, which creates several outputs, including a "grid of the longest flow length upstream of each point" (PLEN), as well as Strahler Order raster. I have also executed the Stream Reach and Watershed method (i.e. StreamNet), which also returns several outputs, including a vector of the stream links that also includes a Strahler Order attribute.

While I know I can get a value for the distance to the divide using any cell from the PLEN raster, what I want is just the longest flow from my outlet up to the divide as a discrete result--raster or vector doesn't matter. I'm not seeing a way to derive this result using just the TauDEM methods, which makes me think I'm overlooking or misunderstanding the language describing the proper method.

Is there an existing method in TauDEM that renders the longest flow in a basin as a single, discrete feature/value?

(FWIW, I tried using the LongestFlowPath method from WhiteboxTools just for this one step in the workflow, but it fails to run to completion. At this point I'd prefer to stick with TauDEM for the solution.)

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