Clipping x,y,z point data above or below a terrain layer

in Vector Data
Hi all. Been using Global Mapper for 7 years and should have joined this forum long ago, but this is my first post!
I have a large point data file with z values. I would like to clip this dataset against a terrain layer present in GM as a raster file (.grd format) so that I just get the point data below the terrain layer. I cant see this option as a direct command option. Is this possible? and how?
Thanks
I have a large point data file with z values. I would like to clip this dataset against a terrain layer present in GM as a raster file (.grd format) so that I just get the point data below the terrain layer. I cant see this option as a direct command option. Is this possible? and how?
Thanks
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Answers
You ask a really interesting question and one I've never thought about before. The answer with many things in GM probably is that, no there isn't a direct command, but there almost always is a workaround in GM!
One way that I would do it would be to create a surface (GM Grid) using your point files and then using the Analysis, Combine/Compare Terrain Layers select the "Subtraction (Difference) - Signed" Operation to create a third surface (or GM Grid) reflecting point z values above (+ve) and below (-ve) your original terrain layer. I'd then contour it at zero value (i.e. tick box for "Only Generate Contour Layers at Specific Height) and this will give you a line separating points that are above and below the Terrain layer and can be used to select the point data you are after. I have attached screen garbs of an example below:
This is just how I would do it. I'd be interested to hear from others how they would be it.
Cheers,
/al
Have you tried exporting your point files from GM in vector form as "XYZ (Simple ASCII) Text File" and then importing it back into GM and creating a surface (Grid) that way? Worth a shot. That's what I did in my example above.
Alternatively you could go "old school" and, if the comparison surface or terrain is flat or reasonably so, you could export your points as a simple text file or csv, import into Excel or text editor and search for all points above or below your target surface elevation. Obviously this won't work if you have a sloping or irregular comparison surface.
Good luck!
Cheers,
/al