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
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.
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
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.
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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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.
Well there's the problem.
BTW, pond 2 has a mathematical volume of 655.41 cy.
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.. 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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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/
Nice write up
neilyj (No connection with Autodesk other than using the products in the real world)
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