Global Mapper Pro

Watershed analysis - streams to top of watershed/catchment?

Hi there,

I am a big fan of the watershed analysis tool. I was wondering if there is a way of getting the streams generated to start at the top of the catchment/watershed? I am looking to set up a rainfall-runoff model which requires mainstream lengths indicating the longest flow path in a watershed/catchment.

I have tried setting the minimum sub basin size very small and also to not discard any stream lengths.

Any tips would be greatly appreciated.



Best Answer

  • JSL
    JSL Global Mapper User
    Accepted Answer

    You could try this:

    1) Create your upstream watershed areas

    2) Select the watershed areas and use Advanced Feature Creation Options to create lines from the areas

    3) Select the lines and run the Analysis/Measurement->Calculate Elevation/Slope stats process, selecting the option to create points at the maximum and minimum elevations.

    4) Select the maximum elevation points of the outlines and rerun the watershed process, choosing the option to Trace Flow from Selected Points. You should end up with flowpaths that extend from the highest point of the watersheds down to their pour points and beyond to the extent of your workspace.

    This worked when I tried it, but I can't get GM to upload my screenshots into the comments.


  • Thank you, that sounds like pretty straightforward process.

  • I've had a bit of a think about the above approach, and while it seems logical at first, I can think of instances where the highest point on the watershed boundary may not necessarily also be the furthest point from the watershed pour point.

    In hindsight I should've been clearer in my question, but I am trying to find a way of easily figuring out the longest flow path in a watershed.

  • JSL
    JSL Global Mapper User Trusted User

    Here's another option:

    Create the upstream watershed areas

    Create lines from the watershed areas

    Create points spaced apart at your desired distance on the lines

    Select all the points and rerun the watershed generation process

    Select the actual watershed areas and then run Select All lines within the Area, using the option for any overlap with the selected area, then crop the lines to the areas

    Run the measurement tool on the lines and find out which is the longest. They should already have the label of the watershed as an attribute.

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