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Export elevation difference

rayjoslyn Global Mapper User

I have a loaded 3D point file and an elevation grid loaded.  I want to export a report (text file) of the elevation difference between each point and the elevation grid at that point.  How can this be done?

Best Answer

  • geomannie
    geomannie Global Mapper User
    edited January 2017 Accepted Answer
    Hi rayjoslyn

    You can but its a bit fiddly.
    1. Open your grid and point files.
    2. Use the digitiser tool to select all the points
    3. Right click and select "Analysis/Measurement" and then "Apply Elevation to Terrain Layers to Selected Features", select the appropriate option. You will now have a new attribute field for each point which is the grid elevation.
    4. In the Control Centre, right click on the point data entry and select "Layer" and then "Calculate/Copy Layer Attribute Values". In the next dialogue make sure that the point data set is selected.
    5. Use the simple calculation and subtract your source attribute (e.g. grid height) from the attribute value (e.g. point height). This will write out a new data field to the point features of the height difference.
    6. Export a CSV file (under "export vector/lidar" option) including "attribute values" option ticked, and you will have the height difference value as a field.

    I hope this helps.



  • rayjoslyn
    rayjoslyn Global Mapper User

    Using this approach I found that my points file already had an elevation field in it, so when I apply the elevation terrain to the points it would either add or overwrite my existing elevation field value.  So I had to copy the elevation field to another field then I could apply the elevation from the terrain layer so that I have both the original (point height) and the elevation (grid height) to do the comparison on.

    My next step was to try to script this, but I cannot figure out how to get the COPY_ATTRS command to copy an existing field to a new field within the same file.  Is this possible?

  • Mykle
    Mykle Global Mapper User Trusted User
    A different approach may be worthwhile.  I use this procedure to compare DEM elevation data with field GPS values.  

    Open the options dialog for your point data file.  On the tab "Feature Types/Labels/Elevations" look at the option "Get 'Elevations' From Attribute Value".  It may be set to <default>.  Change it to <Don't Get Elevation From Attributes>.  

    Export your point data file: Export > Export Vector/Lidar format > CSV.  In the CSV Export Options dialog, select the Options tab, and check "Include Elevation Values for Each Point".  

    Open the CSV file in a spreadsheet program.  Note that you have more than one column for Elevations (or similar names).  One should be your point data elevations, and another should be from your elevation grid.  Add a column and subtract one elevation column from another.  

    I have not played with scripting this process.  

  • This is a much simpler process. But now I want to import the diffs and plot dots with colors indicating the diffs. Any suggestions?
  • rayjoslyn
    rayjoslyn Global Mapper User
    I've been playing with this.  I've been using the create density grid (heat map) from the points using a delta z value  (Elevation-GPS value).  See settings attached:
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