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# Volume Survey Uncertainty

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Posts: 1500
Member
(@jaro)
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Joined: 9 years ago

For a site 40m x 100m, I would just get out the total station and check it. I don't think it is necessary to check it every time but enough to get a good feel for the actual accuracy you are getting.

The thing I have a hard time getting across is the error for each layer compared to the thickness of the volume. If i have 0.10m error in the original layer, then 0.10m error in the final layer, but the vertical difference between the two layers is 10 meters, Then your error estimate would be 2% which is pretty good.

0.2 meter total error when the total average depth of the volume is 1 meter, then your error estimate would be 20%.

Posts: 6214
Member
(@mightymoe)
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Joined: 9 years ago

I'm with JaRo on this one.

SOP for all our photo topo jobs is to check points.

Unless this isn't a job where something is getting built.

Posts: 1699
Member
(@leegreen)
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Joined: 8 years ago

To get an accurate check, you need to measure the site with a different procedure and different equipment (our spec's call for equipment 3x more accurate than published accuracy), then calculate the volume with different software and COMPARE results. You will be surprised. I'd expect your quantities to vary by ±4% or more.

At least run the field data through a second software. They all calc volumes slightly different.

Posts: 94
Member
(@rover83)
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Joined: 4 years ago

Agree with Lee. The only way to get a quantifiable standard error are independent measurements of a higher accuracy, per NSSDA specs.

However, methodology for quantifying the error aside, for the final volume you will be computing the combined error for a sum of quantities.

If there is a 0.1m vertical standard error in two 4000m2 surfaces, that is a 400m3 standard error in volume for each. The difference between them is a sum (subtract one from the other).

The combined error of a sum is the square root of the sum of the squares of the standard errors, which yields approximately 566m3.

Member
(@millshot259)
Joined: 3 months ago

5+ posts
Posts: 6

@rover83

Yeh so i have RTK obs on the surface which have a higher vertical accuracy which i will use for some checks.

But

Maybe I should of explained the situation further. I used photogrammetry to compute a stockpile survey. The one

survey was used to determine the base surface and the top surface therefore i have determine the two surfaces to have the same

uncertainty. The uncertainty in each surface is made up of the GCP uncertainty and the photo - software processing uncertainty. combined = +/-0.071m.

The relative uncertainty between the two surfaces as you have said is the square root of the sum of the squares which computes at +/-0.1m.

so with that said my question really was if people thought it would be okay to use the relative uncertainty between the two surfaces to compute a total volume uncertainty. which would look like

volume uncertainty  =4000m2  * 0.1 = 400m3

as a percentage  400m3 / 12500m3(total volume) = 3.2%.

or is there another process such a uncertainty of a mean which is used where the mean is the relative uncertainty? as its a +/0.1 will there be points that will cancel each other out and approach 0 and if this was a problem is there a equation to compute the real average uncertainty??

Thanks

Posts: 716
Member
(@jp7191)
500+ posts
Joined: 9 years ago

Don't worry about it.  No matter what you do some guy named Jed with one of these [attach]4898 is going to call you wrong based on his truck count.  That was typically my experience.  Good Day, Jp

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