Global Mapper v25.0

Extract water level/DEM intersection area

I have a DEM and I want to extract an area feature (or line if necessary) that is the equivalent of the intersection of the DEM and the water level as set in the Configuration menu.

What I'm looking for is essentially the red line (I just roughly sketched that in), but more precise. Is that possible (running GM Pro v24.0)?

Thanks in advance,



  • Hi David,

    I feel your pain! I have asked the same question on this forum previously (twice) and as far as I know there is currently no way of creating a line or area at the intersection of two surfaces directly within GM. There is a workaround that works partially at the link below, but would be awesome if it could be done directly without all the stuffing around.



  • BillB
    BillB Global Mapper User Trusted User

    Would it be sufficient to create a contour line at the specific water elevation?

    You can specify the minimum height at the level you want, check 'Start at minimum elevation...' and set the maximum height to be less than the contour interval, so that a second contour line isn't created.

  • Hi Bill,

    I agree that its a piece of cake if one of the surfaces is flat (water surface in this case) and you can create a contour along the intersection, but if both surfaces are sloping there is currently no way I know of to directly create a line or area along the intersection within GM. The examples I'm thinking of is to create the "toe line" where a fill pad intersects sloping ground or the "crest line" where a pit or excavation merges with sloping natural ground surface etc.

    I wish there was a way to do this directly, but I haven't been able to find it and have to resort to contouring the signed difference combined surface as explained in the link in my message above. I really wish the GM folks could add this as a feature...

    If anyone out there knows how to do this directly with GM, please let us know!



  • 946
    946 Global Mapper User Trusted User

    Just do this:

    where 243 matches the water level set in Vertical Options


  • As @946 points out, it is easy if the intersection is with a plane.

    But @CarrickCon mentions that the problem sometimes is intersecting two different surfaces.

    The answer may be to create a third surface using the Combine/Compare Terrain Surfaces tool with the subtraction method. The resulting surface can be used to create a contour at the "zero elevation". Not really zero in the normal sense, as in this case it represents where the two surfaces had identical elevations.

  • Hi Ron,

    Yes I already use the "zero elevation contour third surface" method that you mention above, but with mixed success, particularly if one of the surfaces is a complex topography.

    Would be great if the GM folks could build in the ability to delineate the intersection of two (or more) sloping planes with a line or area. This would be really useful to delineate the crest-line of excavations, ponds, pits etc. and the toe-line of above ground fills, pads, roads, bunds etc. Come on Blue Marble you guys can do it!

    If anyone else would like to see this feature get on board! 😉