There is a discussion in our office at the moment regarding the rounding off to 'X' (depending on size) significant figures for Volumetric reports. This ranges from a couple of hundred m³ stockpiles to massive 1/2 yearly reports at land fill sites.
Arguments for include absorbing assumed inaccuracies and if you show down to the last m³ you're showing far more certainty into results that you can realistically claim.
Arguments against include that during a QA you produce the report showing final amount anyway and that rounding errors can sometimes be larger than the difference in survey. (I.e. if you round to 50m³ Surveyor A gets 123, Surveyor B gets 129 and a 6m³ difference turns into a 50m³ difference. @100m³ it's significant but at 100,000 it's less so.)
I'm interested to see how other Surveyors out there handle it, what kind of disclaimers you put on your volume surveys (if any) and if you do round, how many significant figures for what size pile.
I just use sig figs, and estimate how many I have on all of my measurements. For instance if I am grabbing the side of a big uneven pile, I'm not as accurate as the gun thinks, but I might decide I'm within a tenth, or just to the unit, or whatever. That solves for your problem of things being significant when they are smaller digits and less so when they are larger ones. 2 sig figs at 100 is within 5 cubic units, while 2 at 100,000 is within 5,000.
Aerial photogrammetry is correct to actual volumetric determination with a 6-inch focal length lens and flying at 1,500 ft above terrain to plus or minus 2%. So, whatever the volume turns out to be, ... rounding off to aprox. 2% of volume is realistic.
Agree with the professor on the 2%. However, for a value presented in a report, I'd state the volume with no rounding but include with it "plus or minus 2%." This gives the reader sufficient information for evaluation or subsequent actions.
I'd caution a 2% disclaimer on any stockpile.
Unless I missed it you didn't say your method of survey.
The base has significant bearing on your volume.
If you create a 'plaster cast' survey then maybe 2% is okay providing the base has been surveyed before.
If you are prepared to defend 2% then make sure your methods are sound.
The sides of any stockpile can have imperceptible dips or bulges that affect the outcome.
Do a quick calculation of a side and allow 4" dip or bulge. Try 6". They can be misleading.
I've surveyed many stockpiles, holes etc and we would suggest for stockpiles 10% was a safe figure.
Quarry owners etc never had a problem with that.
The volumes are 'loose' in stockpiles, and whilst that's obvious to most, I also state that in my report.
I round to nearest significant figure and leave it at that.
You could spend twice as long and cost the client more and getting more points lowers errors. Whether the cost is justified depends on the owner and his desires.
Should have explained a little more in my post. My point was to provide the reader or client with a number and then tell him or her how accurate the number is. If the uncertainty is 10%, then the volume should be reported "plus or minus 10%." If uncertainty is 5%, then "plus or minus 5%."
When a volume is expressed "plus or minus 2%," then I'd assume all variables have been accounted and that the result is what is stated.
Yes fair comment Alan.
I think we are addressing similar but different aspects of the volume /stockpile matter.
