The Calculate Volume menu item is used to calculate the volume of an intersection formed between two surface objects (either a grid or a mesh) and a region that acts as a boundary slicing plane that surrounds the two surface objects. The results of the calculation are displayed onscreen in a popup message, and are also added to the Message Log.
Volume Settings
These options control the volume calculation.
Top Surface:  This option selects the grid or mesh to be used as the top surface in the volume. 
Bottom Surface:  This option selects the grid or mesh to be used as the bottom surface in the volume. 
Boundary:  This option selects the region to be used as a boundary slicing plane that surrounds the two surfaces selected above. 
Overlapping Volume Option:  This option controls the behavior of the volume calculation when surfaces overlap. The option 1. Positive only includes positive volumes, where the top surface is above the bottom surface. The option 2. Negative only includes negative volumes, where the top surface is below the bottom. The option 3. Absolute Sum includes both positive and negative volumes, added with the sign ignored. The option 4. Signed Sum includes all volumes, but negative volumes will be subtracted from positive volumes. 
Volume Error:  This option controls the volume error bounds. The calculation is performed as a set of vertical columns summed together with an upper and lower bound; if the difference between the two bounds is too large, the calculation is repeated with more, thinner columns, until the desired error limits are met. 
