Global Mapper v25.0

Replacing DEM heights from other TIN grid so heights match up

stihl
stihl Global Mapper UserTrusted User
edited July 2014 in Elevation Data
Hi,

I'm on the latest GM version and I was wondering what the correct procedure is for lowering the heights of a DEM with the values of another TIN grid.

The DEM is an Arc ASCII and has a very high resolution of 5cm per pixel.
A surveyor measured individual heights in between the vegetation, a total of 68 points from which I can generate a low resolution TIN grid.

How do I replace the heights from the high resolution DEM with the values of the low resolution TIN grid but still keeping the high resolution.

The combine/compare feature hasn't really worked out for me since I either end up with a TIN grid that has the resolution of the DEM or a DEM that has the resolution of the TIN grid. But I can't seem to simply lower the heights in the areas where the DEM and TIN overlap with the DEM being on the same height as the TIN.

If needed I can export the DEM as an XYZ pointcloud if this provides easier editing options.


Thank you for your time.

Comments

  • global_mapper
    global_mapper Administrator
    edited July 2014
    You should be able to create a grid from your points using the Analysis menu command and specify to create it at the same resolution of the DEM layer. Then so long as that is on top of the existing DEM it should replace the DEM where it exists. If you want to smoothly blend it in, go to the Feather tab of the Options dialog for the TIN grid layer and select to feather to the coverage of the layer. If you have a polygon that better represents where you want to use the TIN, you can feather to that boundary instead.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • stihl
    stihl Global Mapper User Trusted User
    edited July 2014
    Hi Mike,

    Thanks for your explanation. I've gotten closer to what I wanted but not quite there yet. I made a DEM with UAV photogrammetry of a hill with vegetation ontop. I've gathered RTK GPS survey points measured inbetween the vegetation giving the bare ground height.
    For simplicity; for the high resolution DEM I'll refer to DEM1. For the lower resolution DEM I'll refer to DEM2

    What I did was place DEM1 ontop of DEM2 in the layer list, then ran the 'Minimum elevation' from the combine/compare menu.
    I then ended up with DEM1 which had DEM2 ontop of it in the overlapping areas of the two DEM's and in the areas where DEM1 was higher situated than DEM2. In the areas that weren't overlapping, it was showing DEM1. Exactly what I wanted.

    Unfortunately the peak of DEM1 isn't in the overlapping area which causes a very high and steep height difference from the peak of the hill to a lower height area. Is there any way to interpolate this height difference so it's a smooth hill?

    Ór is there any possible way to lower the Z values of DEM1 so the vegetation height roughly equals the Z values of DEM2?
  • stihl
    stihl Global Mapper User Trusted User
    edited July 2014
    I've tried feathering. But it just feathered the outbounds of the entire DEM. If I used a poly around the area of interest it feathers those edges but it also crops anything outside the outline.

    Here are three images showing exactly what I mean:

    High resolution DEM1:
    http://static.dyp.im/ic2rH9xo5A/fe58b62b59df24223c38f1a557c06435.jpg


    Low resolution DEM2 (lower Z values):
    http://static.dyp.im/kgndlxij32/ead7bbbe679e3e54ea4b3cae6dabbf52.jpg


    Combine/compare 'Minimum elevation' of the two DEM's:
    http://static.dyp.im/SqzrxUwM1o/2bf838583c9cfd4c45a1da77dd91d820.jpg
  • global_mapper
    global_mapper Administrator
    edited July 2014
    Does the coverage/feather polygon have a hole in the middle where you don't have coverage? I suspect the auto-generated coverage polygon will cover that gap in that middle so it won't look like an edge. If you manually draw the areas with the Digitizer Tool you can then feather to them and make sure the blending happens exactly where you want it.

    Thanks,

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