Composite Volumes / Average End Area

[cdg-desirea]

Can someone direct me to information showing the difference between composite volumes and volumes obtained from average end area?

[Anonymous]

Hi,

An average end area volume computation is the "hand calculation" method for
computing volumes for objects like roads where the length of the object is
large compared with its depth/width values AND there are enough sections
taken so that variations in the depth and width are reasonably sampled.

A composite volume is the volume computed between two Triangulated Irregular
Network (TIN) surfaces as used by nearly all Civil Engineering software.
It's computation is so complex that it is only feasible with computers. The
results of the computation are "exact" for the differences between the two
models and to the extent the models represent real world surfaces accurate
for the real world surfaces.

The composite method is better suited to volumes where the length and width
of the area of interest are of similar magnitude, or where the depth varies
irregularly such as you may get in an open cut mine.

The methods will give different answers in all real world situations. Which
you should adopt if you have both is a matter of judgment.

Obviously both are sensitive to the amount of survey information you have on
the existing ground and you should NEVER assume accuracy better than
warranted by the survey data.

Lastly, in real world situations when you move earth, it bulks up by
varying amounts and the computed volumes need adjustment to allow for this
if you are planning for the "in truck" volume to move. The variation in
the "bulking factor" is usually so great that the results of the average end
area/composite computations become meaningless when quoted at better than 2
or 3 significant figures.

--

Laurie Comerford
CADApps
www.cadapps.com.au
www.civil3Dtools.com

wrote in message news:5660685@discussion.autodesk.com...
Can someone direct me to information showing the difference between
composite volumes and volumes obtained from average end area?

[cdg-desirea]

That helps a lot. Thank you!

[Anonymous]

With some software packages you can specify a line and they will run cross sections at a nominated interval cross your surface at that direction, and calculate a volume that way, regardless of the shape of your surface. Or you can cut your surface into horizontal sections and get a volume that way. If you specify a close enough interval (1m is more that good enough) the end area and composite volumes calculates are surprisingly close, agreeing with a couple of cubic meters or better than 0.1%. Because the calculation approaches are fundamentally different, it is a useful check as to whether the software is getting a correct volume.

As Laurie said, the reliability of the surveyed surface is what ultimately determines the accuracy of the volume.

With the general bugginess of Civil3D I honestly wish there was an alternative volume calculation method included. I have exported TIN surfaces to other software packages though and arrived at very similar volumes every time, in case anybody wants to know.

[Anonymous]

I appreciate you sharing what you've found. I have done several comparisons
with Civil3D '06 and another app, and found very tight results.

As for another method, have you tried doing section volumes? While the data
would be coming from the same source, the section volumes gives a nicer
visual, particularly for 'old-timers' that are used to that methodology. (I
haven't done this with '08, but imagine it is at least as simple as it was
with '07.)

wrote in message news:5661041@discussion.autodesk.com...

With the general bugginess of Civil3D I honestly wish there was an
alternative volume calculation method included. I have exported TIN surfaces
to other software packages though and arrived at very similar volumes every
time, in case anybody wants to know.