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.