Gerry, could you expand on this a bit. If you changed the z-factor, will the output units be in the z units rather than the xy units? The tool documentation is ambiguous, but led me to believe the volume would always be in the xy units (the only evidence is the statement "This is typically used to convert Z linear units to match XY linear units.").
You would change the z-factor if your XY coordinates are in different units than the Z. For example, if your data is in a horizontal projection using feet and the vertical datum is in meters. The tool would interpret a 1 x 1 x 1 cube as 1 cubic feet, when it should actually be 1' x 1' x 1m or 1' x 1' x 1' * 3.28084 (the z-factor). I assume you would input 3.28084, although it may be 1/3.28084. Luckily, I work in Canada, where the only unit is meter (or metre!).
Thanks for your prompt reply Darren. I understand when to change the z-factor, but my question is about what a change in the z-factor does to the output volume units. In my case, I'm working with a DEM that has units of m x m x cm (the standard format for NED DEMs), so I think I specify a z-factor of 0.01 m/cm (though now your answer confuses me about the appropriate value). Assuming that's all correct, my question is: Is the output volume measure in units of cubic meters, or cubic centimeters? I had assumed it would be cubic meters, but the values seem way too large to be correct, and dividing by 1e6 cm3/m3 seems to make them more reasonable.
The output values will be in unit of measure of the projected data. If your raster is using UTM zone X, the volumes are areas output by the tool will be in (assumed) m^3 or m^2. Failure to convert the cm z values to meters will result in calculation errors.