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Volume calculation - different results

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Message 1 of 4
watersam
449 Views, 3 Replies

Volume calculation - different results

I'm getting different results for cut/fill, depending on which surface I make my base and comparison surface.  My surfaces are fairly simple, with no vertical overlap, except for possibly along the edges.

 

I'm trying to determine the volume of an existing ditch.  I've got an EG surface, created from a 1m resolution GIS raster (created with surface from DEM).  I created feature lines on either side of the ditch, inherited the elevations of the EG surface, and added them to a "lid" surface.  When I use the "lid" as my comparison surface and the DEM surface as the base, the volume calc is 842.66 cu. m fill and 0.01 cu. m cut.  When I reverse the surface order, I get 924.38 cu. m. cut and 0.01 cu. m fill.  I've checked the surfaces--no spikes. 

 

Can anyone explain this?  I would think that the volume calc would return the same value for volume either way it was calculated, the only difference being whether it was cut or fill.

 

Thanks,

Sam

3 REPLIES 3
Message 2 of 4
neilyj666
in reply to: watersam

You certainly should get the same volumes but with cut fill reversed - can you post the drawing??

neilyj (No connection with Autodesk other than using the products in the real world)
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Message 3 of 4
watersam
in reply to: watersam

Here is the drawing.  I tried this again with another drawing (I've got more than one ditch to fill) and had similar results.

 

I'm working in C3D 2011.  The two surfaces are "Grid from DEM file w001001" and "Delph_R_DS"

Thanks for looking at this!

 

Sam

Message 4 of 4
watersam
in reply to: watersam

As a test, I created 2 volume surfaces: "Vol_check" uses the "lid" as the comparison surface and "volcheck2" uses the "lid" as the base.  They both have the same contours, but Vol_check Vol_check's values are positive, while volcheck2's values are negative.   I think I found the problem though.  Vol_check has areas of discontinuity around the boundary of the surface (little triangular nibbles out of the surface), while volcheck2 has no discontinuities.  I think I know which result to trust now. 

 

I'm not sure why this would occur with one surface calculation and not the other, so I've still got that question.

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