Volume measurments - height above ground or sea level?

Hi all

I am trying to extract a 1m profile volume through a stockpile centre. Can anyone clarify the requirements around "base heights" in the Analysis/Measurment > Measure Volume (cut fill) command.?

If I draw a simple straight line through the stockpile (with diameter set to 1m), do I have to extract heights to the line / lines vertices before I use the Measure Volume command for the volume calculation to be right?. Essentially I only require the volume within the 1m line from the ground to the stockpile height. The ground, while relatively flat, does sometimes vary in heights between the stockpile ends. Would I need to create a ground surface (DTM) first to extract heights of the ground or will Global Mappers algorithm do that? Also, does one simply work with the heights above sea level or above ground level?

Any advice will be appreciated! 
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