Volume Survey Uncertainty
I am wondering if anyone could me with the topic of 'volume uncertainties'.
Basically, I have computed a relative uncertainty between two DTM surfaces derived from photogrammetry of +/- 0.10m. Now using an area, I can calculate a ‘thickness’ error between the two surfaces to compute a volume uncertainty.
But I have been led to believe as the survey contains thousands of points generated by photogrammetry there is as many points that are +0.10m as there are -0.10m and maybe cancel each other out and approach 0.
Is this where the ‘uncertainty of a mean’ is used? I am confused can someone shed some light on this. I am using approx. 25000 points and a relative uncertainty of +/- 0.10m over and area of approx. 4000m².
Volume calculations are at most an approximate value, to begin with.
The elevations are not exact and no two gatherings to collect the required data will match.
In a nutshell, it is a tool to achieve the best value possible for an amount of dirt or other material that will change in size.
The entire volume is a variable.
Is that 4,000 meters by 4,000 meters (3,950± acres) or 207.5 meters by 207.5 meters (10.6± acres) for us non-metric old fuddy-duddies?
The problem is; there is no guarantee, the uncertainties, the random errors, will cancel each other out. That's why it's variable; plus or minus.
The report provided, along with volume calculation, should provide the method used to create the surfaces, and the calculations that derived the quantity. Assuring that the reader understands that is a high and low number for the actual size.
This reminds me of a project @Roadhand was working on. A big highway job, somewhere deep in the heart of Texas. He was wondering about the best way to check the accuracy of his equipment. It seems that the Bean Counters figured out that an eighth of an inch of concrete, over the entire project, was worth a million dollars; they wanted someone to provide a quantity, to that accuracy, so that an equitable pay out would be provided. I told him to tell them that a sixteenth of an inch was worth $500,000, why weren't they chasing that? There were some nicer, less condescending answers, that quickly pointed out; it would cost a lot more than 1 million dollars to provide that level of accuracy.
Accurate results can be achieved, but it's a tedious, time consuming task and as we all know; time is money.
Why are you trying to determine this? Is it a part of the deliverable to the client?