Global Mapper v25.0

subtracting DEMS

marten
marten Global Mapper UserTrusted User
edited November 2010 in Elevation Data
Is there a way to subtract one DEM from another in GM?

Thanks,
Marten

Comments

  • marten
    marten Global Mapper User Trusted User
    edited October 2010
    marten wrote: »
    Is there a way to subtract one DEM from another in GM?

    Thanks,
    Marten

    I read a little further:

    "Support for combining/comparing terrain surfaces, such as DEMs. This allows you to generate a new terrain surface by combining/comparing two loaded terrain surfaces with operations like subtraction (difference), averaging, and min/max. The results can then be exported to any of the supported elevation grid formats, such as USGS DEM"
  • global_mapper
    global_mapper Administrator
    edited October 2010
    Marten,

    Use the File->Combine Terrains menu command to do that.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • jessie007
    jessie007 Global Mapper User
    edited October 2010
    Hey all. I'm new here! I'm glad to find this online group because I see that I have a lot of these similar questions. Anyway, thanks for the help and I look forward to connecting with you all and collaborating.
  • Matt
    Matt Global Mapper User Trusted User
    edited October 2010
    With regards to Combining Terrains. Would any of the options choose the value with smallest sample spacing for the new layer. For example, I'm trying to merge 3 different sets of terrain data that overlap in some area, but not others. I want to combine them in such a way that the most detailed available for any particular area is used. I've tried layering them in different orders, adjusting the altitude ranges, etc, but can't quite get it the way I need it.
  • global_mapper
    global_mapper Administrator
    edited October 2010
    Matt,

    If you just arrange your data so that the most detailed data is on top of the others (i.e. make it down further in the Control Center), then sampling should come from that where available automatically, you shouldn't need to do anything special as the topmost layer at any location is always used.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • marten
    marten Global Mapper User Trusted User
    edited October 2010
    The subtraction of surfaces is super easy now. Thanks a bunch.

    Marten
  • Matt
    Matt Global Mapper User Trusted User
    edited October 2010
    I've tried putting most detailed on top. But doesn't quite work. Here's the problem. I have 2 layers.

    - Layer 1 is low detail bathymetry and ground elevations
    - Layer 2 is high detail ground elevations, but has an elevation of 0 in the water areas.

    If I put Layer 2 on top, the ground looks right, but it hides the bathymetry. I've tried adjusting the detailed layer to not display data below sea level. But then areas of ground below sea level show the low detail data.

    The only thing I can think of is to somehow crop the detailed data to remove the zero elevation data in ocean areas, but haven't figured out a good way to do that yet that won't take weeks to process. Was hoping there might be another option.
  • global_mapper
    global_mapper Administrator
    edited October 2010
    Matt,

    There is an easy solution. But 'Layer 2' on top, but go to the Adjust Elevations tab of the Options dialog for that layer and change the min/max elevation range so that it stops just above 0 (say 0.1 meters as the minimum). Then the 0 elevations should be treated as invalid and the underlying bathymetry will show through.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • Matt
    Matt Global Mapper User Trusted User
    edited October 2010
    I tried that also. That method works great except for land areas that are below sea level. Those areas revert back to the low-detail data. But that is good compromise and probably the method I'll end up using.
  • GavinMartin
    GavinMartin Global Mapper User
    edited November 2010
    Thanks for the informative post. I also had the same query and your post helped me a lot in solving issue.