I've just come across a blog post highlighting a major issue with cut / fill calculations performed by revit on topo surfaces. I'm really blown away by this and slightly concerned having just issued a cut / fill schedule on a job.
I've remodelled the example shown in the blog I read and confirmed the error. And its a biggy.
The pads are all 1m deep. The volumes of the pads themselves report correctly.
All pads are set with a -9m offset to create a 10m excavation.
The volumes only report correctly for 10m3 of cut. All examples above that have random and potentially massive errors.
Come on Autodesk. This needs a urgent fix!
Solved! Go to Solution.
The cut and fill volumes calculated by Revit are approximate. When you test on a very regular surface/shape like this it is very easy to find descripancies in the "correct" vs. the approximate value. On an irregular surface doing the calculation manually you would end up aproximating the calculation as well, and would probably come up with a value closer to what Revit will report.
OK. So a crucial bit of information required. To what level of accuracy are the volumes calculated? Can I write a disclaimed that says "cut and fill volumes are approximate and accurate only to ????? m3" ??
I can't say for sure. This is somewhat dependent on the topographical features, where they fall on the site, size of the site overall. I would say typically the approximation is going to be within +/- 1-2%.
In your example it looks like it is about 1% over.
I'd be happy with that as an answer as long as I can have confidence in the 98-99% accuracy. I just can't help wondering why it can't do the simple numbers in a simple example with 100% accuracy. Its quite unsettling.
I just can't help wondering why it can't do the simple numbers in a simple example with 100% accuracy.
Short answer, the math/algorithim does not work that way.
The long answer, http://en.wikipedia.org/wiki/Monte_Carlo_method
Simple volumes of platonic solids are easy to calculate using memorized formulas. My 7th grader at home can do them. An irrgular shape like a topographical surface is a much harder problem to solve.
Thanks for the this. Very reasuring that its not just a hideous bug. Panic averted!
I would suggest a note about the accuracy is added to the documentation.
..you may want to get an idea to what accuracy the guys are setting out on site (on a rainy day, with shovels and wheelbarrow or big, beefy front loaders). Are they using electronic setting out - is this for a nuclear powerplant of a basic commercial slab?
That would give you a sense of whether there's an issue or not. The last time I cut and filled foundations...it was messy, ball-park work..with a tolerance of around 20%
thanks for the patronising comment. If it were just for a slab it would not be an issue. It is however for a huge volume and as the errors were seemingly random it needed asking to check if it was indeed a bug or other serious underlying fault which meant I could not trust the software.
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