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

oztodd

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

CarrickCon

Hi Oztodd and welcome to the forum!

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

CarrickCon

CarrickCon

Sorry, steps/image as follows:

oztodd

thanks Carrick, I'll give this a go 

oztodd

hi again. I don't think this will work as my point file is like a point 'cloud' which cant be gridded and represented by a terrain layer where 1 x,y node represents  just 1 z value. Not sure if the image shows it well enough, bit a 3D of many black points above and below a terrain surface, ie some in thin air I want to get rid of, and the rest below ground

oztodd

doh!

CarrickCon

Hi Oztodd. I'm not sure where in the World you are but I'm in Perth, Western Australia so likely a different time zone from you...?

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

oztodd

HI Carrick, thanks for your follow up...small world, I'm in Perth too. I think this is too much of a 3D problem for GM so have asked a friend who has Deswik and Vulcan to do. Cheers