Global Mapper v25.0

Carve a ditch using a line?

Simon
Simon Global Mapper UserTrusted User
edited November 2014 in Elevation Data
GM14:
I have a high-resolution DTM to which I wish to add ditches. I have digitised lines that represent these ditches. I want to give the lines widths and depths and then subtract that volume from the DTM.
I have tried the following approach:
-digitise line
-add elevations to the line's vertices
-buffer the line (to my desired width)
-create 3D grid from the area (the buffer)
-alter elevation of the 3D grid (lower it by my desired depth)
-overlay the 3D grid on top of the DTM (layer-wise)
-export all elevation data
This gives a square ditch profile, but I can live with it.

The obvious question: Is there a more convenient way do do this? Even better, has anybody devised a way of carving out a wedge-shaped ditch from a DTM?

Thank you for your help.

Comments

  • Ice Age Mark
    Ice Age Mark Global Mapper User Trusted User
    edited November 2014
    Simon,


    If you have/get the (free) GEM add-on module to V14 (or update to V16 which includes it), you can use the "pad creation" feature to get your ditch with sloping sides. Use a line (*) and buffer it to create a linear area feature (like you already did), but use the BOTTOM width of the ditch. Then use the pad cut/fill slope feature to set the side slope of your ditch (1:1, 1.5:1, etc.). This should "daylight" the side slope of the ditch at the existing topography.
    Theoretically, you could subtract the DTM with ditches from the original DTM to get the ditch volume.


    It should work based on other things I've done that are similar, but no guarantees with free advice. Good luck. If you have any other questions, just ask.


    Mark


    (*) If you first give the line your desired ditch bottom elevations those elevations should get transferred to the area feature, and you shouldn't need to "lower" it.
  • Simon
    Simon Global Mapper User Trusted User
    edited November 2014
    Mark,
    Thank you for the good advice. The reason for lowering was that my ditches consisted of many polyline segments, so editing each one was time consuming. In your suggestion, I can lower the DTM by [my ditch depth], apply elevations from the lowered DTM to my vertices, restore the DTM and then perform the calculations with sloped sides. That should be quicker, provided that I can get a hold of the GEM add-on.
    Thank you.
    Simon