Global Mapper v25.0

Extracting Trees from DSM

gregleon
gregleon Global Mapper UserTrusted User
edited August 2013 in Technical Support
I have a 1m gridded DSM and DTM dataset. I have used the combine operation to create a DSM-DTM layer. I also have building footprint polygons in vector form. I want to extract only the trees from the DSM and generate both a gridded file and a polygonized version of that tree data. I would like to understand how I can use the building polygons to remove those portions of the DSM, then run an operation to only extract additional pixels that are above a certain threshold( like 2m, remove ground noise step). I tried using the flatten option via digitizer, right clicking on selected polygons but this not seem to do what I thought it was going to do.

Any suggestions are helpful...I was given a process to do this in arcgis but I was hoping to accomplish in GM.

Comments

  • global_mapper
    global_mapper Administrator
    edited August 2013
    I think you are on the right track, but you first need to edit your buildings and add an ELEVATION attribute to create the terrain from. I would assign the ELEVATION the value of the tallest building (or larger even), then create terrain from the building areas. You can then subtract that from your DSM-DTM layer. Anything left that is positive should be non-building surface features.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • gregleon
    gregleon Global Mapper User Trusted User
    edited August 2013
    Mike -
    I was able to create a grid layer of just buildings(used max elev adn this results in a grid of a lot of blank space and elevation values where the buildings are) however when I go Combine/Compare Elevations and pick the DSM-DTM as first and the Buildings Only Grid 2nd and choose difference option it produces an output of only the buildings area grid. It does not seem to be doing the expected calculation, am I doing something wrong here?
    Greg
  • global_mapper
    global_mapper Administrator
    edited August 2013
    Greg,

    Ah yes I forgot that the difference one only creates a new grid surface where both are valid. What you can do is go to the Alter Elevations tab of the OPtions dialog for the building layer and check to replace invalid with 0, then it should generate valid data everywhere within both layer bounds, with the values being the same where the buildings don't cover.

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • gregleon
    gregleon Global Mapper User Trusted User
    edited August 2013
    That worked. Thanks. A few more questoins:
    1. Does the Elevatoin Options - Valid elevation range setting hold true on grid export? Meaning if I have a 2 minimum elevation, will the export only export values above 2? If that does not happen, is there a simple operation to take all elevatoins less than 2 and make them 0?
    2. In regards to the last question, i want to create a binary version of hte tree heights grid, meaning 1 where trees are and 0 where they are not...is there a simple feature for doing that manipulation?

    Thanks.
  • global_mapper
    global_mapper Administrator
    edited August 2013
    Yes the elevation range clamping and other modifications apply on export. It's basically what you see is what you get.

    If you want to get 1 where trees are and zero where they aren't, I would create a custom elevation shader and make the non-tree heights as black and the minimum tree height as white, then you should get a black and white image. You can then export that to a raster file like a black-and-white GeoTIFF to get a binary image.

    Thanks,

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