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Inaccurate grading volumes

11 REPLIES 11
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Message 1 of 12
c.mike96
472 Views, 11 Replies

Inaccurate grading volumes

I have noticed that the value I get when I use the surface volume comparison tool or look at a graded item in the grading group properties box is different than if I calculate it by hand and the discrepancy isn't even consistant.  After spotting this problem in a job I am working on graded two simple ponds to check this.  The first one is 100' x 100' and 5' deep w/ 2:1 side slopes.  Calculating the volume by hand I get 1500 cu. yds but the grading group properties come up to 1506.17 cu. yds.  The accuracy is off by less than 0.5%  The second pond is 100' x 100' and 2' deep with 3:1 side slopes.  The hand calc gets me 654.52 cu. yds. and the grading group properties reads 655 cu. yds.  This discrepancy is closer to 7%.  Could someone help me out?  Is there a setting or something I am missing?  I really need to know if I can trust the volume tools in the program.  Thanks

11 REPLIES 11
Message 2 of 12
Jeff_M
in reply to: c.mike96

I see a difference of 0.5% and 0.07%, the first is about 3 wheelbarrow loads, the second about 3 shovelfuls. How is this not accurate enough for you?

Jeff_M, also a frequent Swamper
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Message 3 of 12
c.mike96
in reply to: c.mike96

On a small scale like the examples I gave it wouldn't be a big deal but on a simple grading like that it should still be dead on.  The problem I am facing in the job I am working on is more serious.  The pond hand calcs at 9,299 cu. yds. but the grading group property reads 3859 cu. yds.

Message 4 of 12
Jeff_M
in reply to: Jeff_M

Also, how did you calc it by hand? My calc for the truncated pyramid (5' deep pond) came out to 1506.17 (using AutoCAD solids).

 

Mass: 40666.6667
Volume: 40666.6667
Bounding box: X: 3845.1604 -- 3945.1604
Y: 8192.1359 -- 8292.1359
Z: -5.0000 -- 0.0000

Jeff_M, also a frequent Swamper
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Message 5 of 12
Neilw_05
in reply to: Jeff_M

Yep, the volume of the truncated pyramid is 1506.17 cy using the formula v=1/3(a^2+ab+b^2)h where a=100' and b=80' (top and bottom of pond) and h=5' (depth of pond). Divide results by 27 to get cy.

Neil Wilson (a.k.a. neilw)
AEC Collection/C3D 2024, LDT 2004, Power Civil v8i SS1
WIN 10 64 PRO

http://www.sec-landmgt.com
Message 6 of 12
neilyj666
in reply to: c.mike96

I agree with Jeff and Neil - I got 40666.7 cuft and 17696 cuft using gradings, two breaklines for top and bottom and the truncated pyramid calculator which convert to 1506.17 cuyds and 655.407 cuyds (not used these units since 1983....!!!) - what's the problem..??

 

The errors you have got suggest  a major operator error in the modelling - can you post??

 

EDIT - did you infill the pond base to complete the surface??

neilyj (No connection with Autodesk other than using the products in the real world)
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AEC Collection 2024 UKIE (mainly Civil 3D UKIE and IW)
Win 11 Pro x64, 1Tb Primary SSD, 1Tb Secondary SSD
64Gb RAM Intel(R) Xeon(R) W-11855M CPU @ 3.2GHz
NVIDIA RTX A5000 16Gb, Dual 27" Monitor, Dell Inspiron 7760
Message 7 of 12
c.mike96
in reply to: neilyj666

The method I used for the hand calc was multiplying the area at half the depth of the pond by the depth.  100x100x5@ 2:1, offset 5' = 90x90 = 8100 sqft, 8100x5 = 40500, 40500 / 27 = 1500 cu. yds.

Message 8 of 12
Neilw_05
in reply to: c.mike96

Well there's the problem.

 

BTW, pond 2 has a mathematical volume of 655.41 cy.

Neil Wilson (a.k.a. neilw)
AEC Collection/C3D 2024, LDT 2004, Power Civil v8i SS1
WIN 10 64 PRO

http://www.sec-landmgt.com
Message 9 of 12
neilyj666
in reply to: c.mike96

Tesselation spacing and angle can affect the TIN and hence the volumes as can swapping triangle edges etc - if any of your volumes are within 1% of what is actually cut and filled you should be very pleased..Smiley Wink BUT as I said you appear to have a major issue in the modelling

neilyj (No connection with Autodesk other than using the products in the real world)
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AEC Collection 2024 UKIE (mainly Civil 3D UKIE and IW)
Win 11 Pro x64, 1Tb Primary SSD, 1Tb Secondary SSD
64Gb RAM Intel(R) Xeon(R) W-11855M CPU @ 3.2GHz
NVIDIA RTX A5000 16Gb, Dual 27" Monitor, Dell Inspiron 7760
Message 10 of 12
BrianHailey
in reply to: c.mike96

Average end area is not the appropriate equation to use for volume calculation for most ponds. Here's my write up on why: http://civil3dplus.wordpress.com/2010/12/23/the-great-storage-debate-average-end-vs-conic/ 

 

Brian J. Hailey, P.E.



GEI Consultants
My Civil 3D Blog

Message 11 of 12
neilyj666
in reply to: BrianHailey

Nice write up

neilyj (No connection with Autodesk other than using the products in the real world)
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AEC Collection 2024 UKIE (mainly Civil 3D UKIE and IW)
Win 11 Pro x64, 1Tb Primary SSD, 1Tb Secondary SSD
64Gb RAM Intel(R) Xeon(R) W-11855M CPU @ 3.2GHz
NVIDIA RTX A5000 16Gb, Dual 27" Monitor, Dell Inspiron 7760
Message 12 of 12
c.mike96
in reply to: BrianHailey

Thanks for all the help

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