Generate Watershed - Drainage Centerlines in flat areas

QiaoQiao Global Mapper UserPosts: 74Trusted User
edited April 2014 in Elevation Data
Hi Mike,

In "Generate Watershed" outputs, we found many cases that the drainage centerlines are "more or less" following shorelines of a lake or the boundary lines of a depression-filled area. See below two screenshots.

Centerlines on input DEM ->Input_DEM.jpg
Centerlines on filled DEM ->Filled_DEM.jpg

In our case, we have to use a very large depression value (100m) in order to get a reasonablly continuous drainage network.

Is there a way to change this behavior? For example, to let the drainage line (more or less) follow the middle (skeleton) of the filled area (or a true water body area). I know that some people are creating artificial slopes to deal with this issue: starting from the first detected outflowing point, gradually increase the elevations by 0.1mm. Similarly can start from any inflowing points and gradually decrease the elevations for the pixels in the filled area (or water body).

Thanks,

Qiao

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited April 2014
    Qiao,

    What you are describing is actually how Global Mapper used to calculate flow through flat areas on the terrain. It slowly increased terrain values from the outflow by a tiny elevation (much smaller than the elevation resolution so no danger of crossing the next real elevation) until there was a flow path. However the depression-filling algorithm now used by Global Mapper automatically creates flow paths as part of the depression-filling process, so once it is finished there are no more flat areas to modify. The new (since v13.1) depression filling algorithm is dramatically faster than the old depression-filling method, but there isn't really any control over where flow paths through flat areas end up.

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • QiaoQiao Global Mapper User Posts: 74Trusted User
    edited April 2014
    Hi Mike,

    Thanks for your reply. That helps. In your new implementation (v13.1 onwards), if the original DEM is flat (e.g., a lake), would that area undergone depression-filling process? Or do you use the artificial slope approach for flow determination in flat areas?

    Do you mind giving us a brief description on how your depression-filling works? I am just thinking whether there is a possible workaround for us.

    Thanks again,

    Qiao
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited April 2014
    Qiao,

    Global Mapper uses the Planchon method for filling depressions (based on paper Planchon, O. & F. Darboux (2001): A fast, simple and versatile algorithm to fill the depressions of digital elevation models.). It's a widely used method for depression filling. You can get the original paper at A fast, simple and versatile algorithm to fill the depressions of digital elevation models - ResearchGate .

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
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