Global Mapper v24.0

Depression Contours

NEV
NEV Global Mapper UserTrusted User
Is there a direct way (yet GM22.1 LiDAR Module) to recognise depresion contours in a DEM or contour set? Any work-arounds? I first asked the question in 2010 (on GM12) and again in 2013 with GM14. I am reliably informed it is possible in Arc and QGIS.
Thanks
Nev

Answers

  • Hi Nev, I don't know a way of doing this directly over a large area with multiple depressions, but there is a "Find local Peaks and Depressions" option within the Contour Generation dialogue box as per attached screengrab. However, this will just find a single depression (or peak) - note that this is different from the "create spot elevation points at min/max elevations" option as the min elevation is not necessarily a depression.

    If it wasn't a huge area (and wasn't full of holes!) I'd probably look at using the Water Display tool under Config/Vertical Options and then selecting the bounding contours for each depression (as per attached screengrab).

    Otherwise I think you could look at running a query on the shortest contour lines, as this would be a characteristic of depressions (but also peaks!) and doing something on that...?

    Anyhow, interested to hear if anyone else has any thoughts/ideas.

    Ta,

    /al

  • Sorry forgot to attach the screengrabs...


  • Syrett
    Syrett Global Mapper User Trusted User

    Depression contours can be done very easily in Golden Software's Surfer. I do it all the time as part of my regular workflow. Export from GM into Surfer, add depression ticks (they call them hachures), export back to GM, done! http://surferhelp.goldensoftware.com/contour/idd_conhach.htm

  • NEV
    NEV Global Mapper User Trusted User

    Hi Syrett,

    Very encouraging reply - thank you. I dusted nearly 20 years' worth of cobwebs off my Surfer 8. Will need a learners permit to run it. What is the expot format from Global Mapper please.

    Best Regards

    Nev

  • Syrett
    Syrett Global Mapper User Trusted User

    Surfer won't add depression ticks to already-created imported contours. What I do is to export whatever grid(s) you used in GM to create your contours. My usual export format is .asc. In Surfer, open this up with the create contours option. Then export the contours as shapefiles and reopen in GM.

    This is just a brief summary. Let me know if you need more details.

    By the way, this is almost the only thing I do in Surfer....my workflow is such that it was worth it to buy a newer version a few years ago (I was also on v8 at the time) and just use it for the depression contours. Global Mapper developers, please take note! If Golden Software could develop this more than 15 years ago, I'm confident you can too. 🤓

  • JSL
    JSL Global Mapper User Trusted User

    This is a pretty complicated workaround, but I don't want to spend $1K on Surfer:

    Generate contour lines on the area of interest

    Separate out all closed contours and run Analysis/Measurement->Calculate elevation/slope stats for the closed contours

    Use Calculate Attributes to copy AVG_ELEV_M of the closed contours to a new attribute, SHORELINE ELEV

    Create areas from all the closed contours

    Calculate the elevation/slope stats for the areas

    Create an attribute that subtracts SHORELINE ELEV from the average elevation of the areas, and all areas with a negative value should be depressions

  • Syrett
    Syrett Global Mapper User Trusted User

    Sounds like you're not interested in the depression contour ticks - just which are depression contours, and which aren't. I create maps that are intended to be used by non-GIS folks, so the ticks relieve a lot of confusion. It's amazing to me how this simple graphic device has fallen into disuse.

  • JSL
    JSL Global Mapper User Trusted User

    Yes, that's correct, I just want to find the depressions. If there's a simpler way to do this in GM, I'd love to know it!