LIDAR points - Height above ground Filterning

GeorgePGeorgeP Global Mapper UserPosts: 6
I am trying to filter some noise out of a lidar dataset and need more details on how the "Height Above Ground Filtering" setting works.  Specifically, what defines the "Ground"  that is being referenced?  Some average of the points selected?  Only ground classified points?  I select points using the digitizer edit tool and then want to unselect out the ones falling say 1ft  below the ground level.   I am trying to do this on a fairly uniformed sloped site - not flat -  and getting very unexpected results depending on the values I enter. 

Any help explaining the intended use of this would be exceptionally helpful.  



Note:  I have found using the automated ground classification routine does not filter noise below the surface because it thinks it is the lowest elevation.  Similarly - playing with the reduce noise routine doesn't help.

Thank you. 
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