Generate Watershed Settings

geomanniegeomannie Global Mapper UserPosts: 208Trusted User
edited November 2010 in Technical Support
Hi

I am intrigued by the generate watersheds tool in GM12 and have tried to use it with limited success. Maybe I am just being too ambitious; this is what I am trying to do.

I have a 90m SRTM dataset over an area of 500x300km in resonably hilly terrain. If I set the resultion to low the generated streams are discontinuous and not really useful. If I set the resolution to mimic the resolution of the DEM, the process never seems to complete (at least not after 10 hours).

What setting might you suggest for creating a realistic watershed on this dataset. How important is the depression fill depth? What are the differences is I set this to 0 or to 25m?

Many thanks

Comments

  • redbeardredbeard Global Mapper User Posts: 279Trusted User
    edited September 2010
    Here's the user manual http://globalmapper.com/helpv12/GlobalMapperHelp.pdf looks like on page 78 is where the watershed tool is. This might help get you started.

    I ran it on a smaller area, and it worked on it for some time, but it did finish.

    My question is,(not to hijack the thread) is there a way to run it to find the watershed above a certain point? (say for judging the risk of floods for a house) ie draw a line across or drop a point on the stream and it generates the watershed above that point?
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited September 2010
    The max depression fill depth can make a huge impact on the time it takes to generate the watershed. Try using a smaller value for that and see if it goes faster. It can take a long time to do depression filling, which is generally more important to a good result on flatter terrain than hilly terrain..

    There isn't yet a way to drop a point and generate the watershed above that point, but I'm planning on adding that in the next week or so.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • geomanniegeomannie Global Mapper User Posts: 208Trusted User
    edited September 2010
    There isn't yet a way to drop a point and generate the watershed above that point, but I'm planning on adding that in the next week or so.

    Thanks Mike

    I'll look forward to your newer version where you can drop a point and generate a watershed above. Perhaps you can drop a note here when you have done that?

    Many thanks

    Bob
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited September 2010
    Bob,

    Yes I will post here when I complete adding the feature.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited September 2010
    Bob,

    I have completed updating the watershed generation to allow the calculation of the watershed area above any point selected in the Digitizer Tool as well as showing the flow from any selected point as part of the watershed calculation process. I have placed a new build at http://www.globalmapper.com/global_mapper12.zip with the change for you to try. Simply download that file and extract the contents into your existing v12.xx installation folder to give it a try. If you are using the 64-bit v12 version there is a new build at http://www.globalmapper.com/global_mapper12_64bit.zip .

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • jpmontoyajpmontoya Global Mapper User Posts: 4
    edited November 2010
    In addition to this feature, it would be very helpful to the the option of drawing watersheds only above some elevation (e.g., 0). For example, I'm working in the coastal zone and would like to map out watersheds that drain into the ocean, but haven't figured out a way to prevent the watershed tool from continuing offshore and constructing watersheds underwater. Perhaps I'm just missing a switch or setting somewhere, but I can't figure out any way to do this.

    Is there any chance of this feature being added?

    Thanks,
    -Joe
    Bob,

    I have completed updating the watershed generation to allow the calculation of the watershed area above any point selected in the Digitizer Tool as well as showing the flow from any selected point as part of the watershed calculation process. I have placed a new build at http://www.globalmapper.com/global_mapper12.zip with the change for you to try. Simply download that file and extract the contents into your existing v12.xx installation folder to give it a try. If you are using the 64-bit v12 version there is a new build at http://www.globalmapper.com/global_mapper12_64bit.zip .

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited November 2010
    Joe,

    What you can do is make the ocean areas invalid prior to calculating the watershed. To do this, simply open the Options dialog for the terrain layer(s), then go to the Adjust Elevations tab and change the minimum valid value to be just above the ocean (i.e. like 0.1 meters). Then your ocean should be marked as invalid and the watershed won't drain there.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
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