My sincerest apologies for causing offence Russ. My advice was on the basis of your use of three decmal places in your reporting of earthwork volumes - something I wouldn't do.
I didn't really. I'm having a bad morning.
I wish the errers were in the decimal places only......
My biggest issue was reading a blog post that said cut/fill calcs are not accurate, then looking hte documentation to see no mentions of the approximations are level of accuracy to be expected and then just hear from ADSK that volumes are approximate only before explaining the 1-2%...erm my head went into a spin and I started to wonder what else revit was deciding to do approximately without documenting it.
As it is I was doing a cut/fill model on a very large hillside to check were were in balacnce so 2% is a lot of material to potentially have to buy in or cart away if discovered later. Granted, its probably the most accurate way of doing it but if you are getting 3 decimal places out of a computer model you might naively expect the computer model to be giving you the answer to within 3 decimal places and doing so accurately. Anexplanation in the help is needed.
At least now I have an answer and can explain likely errors to be expected.
These are the results of making excavations of different sizes, starting from a small 1x1x1 (m), to a very large 1000 x 1000 x 1 (m).
According to these results, I agree with Jeff Hanson's response, above. The variations don't seem to follow a consistent pattern that we can comprehend easily if we see a test as the one shown in that blog that was referenced by Russ in the original post. But we have to keep in mind that the algorithm is made to produce an approximate result not only with simple boxes like in that blog, but also with irregular volumes, which would be the most typical situation.
In this test of mine, with simple boxes of different sizes, the results are always less or just a little more than 1% of the results that you could obtain with simple math. Notice how, for an excavation that is 100 x 100 x 1 m, more than an American football field, the "extra" volume is just 1.6 cubic meters, the 0.02 %, which is less than the actual volume that will result of that excavation because of the "swell factor" of the soil.
I found some incorrect entries in the first column of the topography schedule in imperial units that I posted in my previous message, and since I cannot edit that image in that post, I am posting the corrected version in this new post, for future, correct, reference: