The volume measurements are given under a selected Surface (or set of Surfaces) or Mesh between the XY plane. For example, two Surfaces covering the area between (0, 0, 10), (1,0,10), (1,1,10), and (0,1,10) have a volume above the XY plane of 10 cubed units.
You can adjust the Z value of the XY plane if the plane does not match the actual ground. See XY Plane Height Dialog.
You need to be aware of the coordinate system of the project to get useful volume measurements. For example, if you wanted the volume of a pile of sand you would:
• Mark and reference points on the sand pile surface.
• Process the project.
• Cover the points with Surfaces so the whole sand pile is covered.
• Set up the project coordinate system using scale and rotation (see Importing and Setting Up Coordinate Systems) so that the ground plane is the XY plane (you would need a suitable set of points marked and referenced at ground level).
• Bring up the measure toolbar and select all the Surfaces (made easier by using the Visibility Settings to turn off all features but Surfaces).
The end result would be a volume measurement of the sand pile in project units cubed.
Note: If the XY plane cuts through the volume, the Volume between surface and XY plane takes into account the surface above and below the XY plane, and the measurement returns the sum of the volumes above the XY pane and below the XY plane in the area defined by the surface (hence the term between the surface and XY plane).