Area Volume Calculation

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Area Volume Calculation

Postby Ron Mak » Thu Mar 27, 2003 8:58 am

I am calculating volumes between three surfaces using the same polygon for the two calculations - ie first I do Surface A - Surface B, then Surface A - Surface C. The 3d polygon is the boundary of Surface A. Both surfaces B and C extend beyond A. I'm puzzled because the area reported by MS is different when I do the two calculations, even though I select the exact same polygon both times. I manually calculated the area and know that one area reported by the volume calculation matches, while the other is 2% less.

Also, in the movies all the points are selected for the TOP surface. I am not doing that becuase I want a "vertical wall" to be created at the polygon boundary. Is this the way to do that?
Ron Mak
 
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Areas

Postby Glen Cameron » Thu Mar 27, 2003 9:44 am

Hard to say for sure without looking at the job. Typically I would expect the area to be the same as well but if I look at the job I may be able to answer this for you.

Please email me the entire job (FLX, MSD and QSB files), and I would be happy to examine the job.

Send it to [email protected] and it will come directly to me.

Cheers,
Glen W. Cameron, C.E.T.
City of North Bay, Ontario
Glen Cameron
 
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Area Volumes

Postby Glen Cameron » Thu Mar 27, 2003 11:23 am

Thank you for email me the job to examine. I was able to find the problem.

Your Mach 26 surface is not covering the entire Slab surface. At the north end of the job it is just a tad short of covering the entire slab surface. This would explain why the area was not calculated correctly - it is based upon the actual area covered within your pline which is smaller than the pline area. The other surface, you called Original, does in fact cover the entire slab surface, and more, and it does compute the total area as expected.

Cheers,
Glen W. Cameron, C.E.T.
City of North Bay, Ontario
Glen Cameron
 
Posts: 1395
Joined: Fri Nov 08, 2002 12:18 pm
Location: Corbeil, Ontario, Canada

Volumes

Postby Richard Sands » Fri Mar 28, 2003 2:45 pm

Nothing to do with the accuracy of MSCAD but one must always bear in mind the real accuracy of the volumes as opposed to that supposedly resulting from a field survey. There are always innacuracies in volume determinations and if one claims to be within 5% or even 10% of the true volume then this may well be misleading. The compacted volume to loose volume can vary considerably (there are theoretical coeficients for this ratio). The more points one picks up the better the result, but if the heap or whatever has irregularities, the underside surface is not truly known then the resulting volume can vary considerably. I always tell the clients the results may be within certain limits. In my many years in roadbuilding the road authority recognised these issues. That said, theory should support itself when it comes to the actual maths, but with out knowing the job I cant make comment on MSCAD voulume determination. This is not meant to throw a scare into anyone measuring irregular volumes but to put some reality into them.
regards
Richard Sands
 

Volumes

Postby Glen Cameron » Fri Mar 28, 2003 7:31 pm

Richard - I could not have said it better.

The actual Tin is only as good as the data collected and the breaklines used. With 2 different surfaces, you are compounding the potential error. With compaction and loose variables, you are compounding that yet again.

What you do end up with, assuming that you have a decent amount of data in good condition, and breaklines that would help define the model, and you use the combined compaction and loose factors as appropriate, is an educated guess that should be within a small tolerance, of which value you then need to try and define to your customer so they are happy.

If you can justify your answer - and they pay your bill - and you do not have any legal issues - then the answer is correct to everyone involved. :)

Cheers,
Glen W. Cameron, C.E.T.
City of North Bay, Ontario
Glen Cameron
 
Posts: 1395
Joined: Fri Nov 08, 2002 12:18 pm
Location: Corbeil, Ontario, Canada


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