I am working with volume measurements of heaps of sand, gravel, etc..:

Based on the terrain profile, we estimate an average custom ground level (as often mounds are next to each other, rendering the 'mean level' option useless)

We then use this to calculate the volume; I've always been using the 'volume above', but my colleague uses 'total volume'

I was using the volume above, because I assume that by using a mean level, you cut of some volume below the level of the actual mound, but add that volume by taking the ground below the surface, above the chosen (mean) surface level, hence ignoring the volume below.

To clarify this I made a sketch:

Brown line = mound of which I need to know the volume.

Black line = surface

orange line = 'custom (mean) ground level'

Purple = ground volume

green = volume below

So am I correct to assume that:

'volume above' = purple + volume of brown line above the orange line.

=

volume of the entire mound, above the black surface line (assuming the mean ground level is chosen correctly)

I am still wondering why 'total volume' is then calculated by subtracting the volume below from the volume above.

Thanks in advance