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# Volume Survey Uncertainty

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Posts: 6
Member
(@millshot259)
5+ posts
Joined: 6 months ago

Hi,

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.

Thanks.

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21 Replies
Posts: 8409
Member
(@a-harris)
5,000+ posts
Joined: 10 years ago

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.

1 Reply
Member
(@millshot259)
Joined: 6 months ago

5+ posts
Posts: 6

@a-harris

Yes i agree but im trying to calculate an estimate of uncertainty within the total volume.

Posts: 973
Member
(@dave-lindell)
500+ posts
Joined: 10 years ago

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?

1 Reply
Member
(@millshot259)
Joined: 6 months ago

5+ posts
Posts: 6

@dave-lindell

something like 40m x 100m Dave.

Posts: 4810
Member
(@dougie)
2,500+ posts
Joined: 10 years ago

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.

4 Replies
Member
(@millshot259)
Joined: 6 months ago

5+ posts
Posts: 6

@dougie

right, so you think its fine for me to leave my estimate uncertainty as i have currently calculated +/0.1m?

Member
(@gquimby)
Joined: 9 months ago

5+ posts
Posts: 11

@millshot259

I think it should be fine unless the client has stated any other specifications they would like to see.

Member
(@dave-lindell)
Joined: 10 years ago

500+ posts
Posts: 973

@millshot259

Well, 40 meters by 100 meters is about 328 feet by 131 feet, say 43,060± square feet. Not quite an acre.

0.1 meter is about 0.328 feet (4 inches), so ±0.33 feet over that almost-an-acre is 12,920± cubic feet.

That's 478 cubic yards, the usual measure of volume here, or about 50 truckloads.

That close enough for ya?

Member
(@dougie)
Joined: 10 years ago

2,500+ posts
Posts: 4810

@millshot259

I think you should look at your scope of work and reassure yourself that the method used to collect the data and make the calculations meets the expectations of the client.

If it doesn't, maybe you need to rethink your method.

Dougie

Posts: 1819
Member
(@just-a-surveyor)
1,000+ posts
Joined: 3 years ago

Why?

Why are you trying to determine this? Is it a part of the deliverable to the client?

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