Cubic Footage of a Tapered Log
Date: 08/07/2002 at 18:42:15 From: Christopher Watkins Subject: Cubic Footage of a Tapered Log Is there an equation that will calculate the cubic footage of a log given the diameters of each end and the length? Currently, I am using Smalian's Formula: (Area1 + Area2)/2 x Length (ft) Area = .005454 x diameter^2 Is there a formula that is more accurate by accounting for the taper in the log? Example: Diameter1 = 9.2 inches Diameter2 = 10.3 inches Length = 102.84 inches Smalian's calculates 4.443 cubic feet. Is this the best answer? Thank you for your time.
Date: 08/07/2002 at 23:06:05 From: Doctor Peterson Subject: Re: Cubic Footage of a Tapered Log Hi, Christopher. A "tapered cylinder" like this is properly called a "frustum of a cone," and the formulas for it can be found in the Dr. Math Formulas FAQ: http://mathforum.org/dr.math/faq/formulas/faq.cone.html#conefrustum V = Pi(R^2+rR+r^2)h/3 For your numbers, this gives V = 3.14(4.6^2 + 4.6*5.15 + 5.5^2)*102.84/3 = 7686 in^3 Dividing this by 12^3, we get 4.448 ft^3. So your approximation is pretty accurate in this case - particularly considering that a log is probably not exactly conical anyway. Out of curiosity, I looked up "Smalian" to see where your formula comes from. It's clear why it would be close, since it is simply the average of the volumes of cylinders with the maximum and minimum diameters. The following page Week 2 Mensuration Notes - Hans Zuuring, School of Forestry, Univ. of Montana http://www.forestry.umt.edu/academics/courses/For202/Week2_Notes.htm explains that Smalian's formula applies to a frustum of a paraboloid, which is slightly convex compared to a cone. It also gives a formula equivalent to ours for the frustum of a cone, and one ascribed to Newton that applies more generally, by using a measurement at the middle to account for the actual curvature of the taper. I haven't looked into just when each of these would be exact. This page gives some more detail you may find useful: http://sres.anu.edu.au/associated/mensuration/BrackandWood1998/ Select LOGS.HTM (Measurement of logs), and then volume. If you have any further questions, feel free to write back. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/
Date: 08/08/2002 at 11:33:12 From: Christopher Watkins Subject: Thank you (Cubic Footage of a Tapered Log) Dr. Peterson, Thank you so much for your time, and for the quick lesson. For your information, I work for a timber company, and a computer scanner at our lathe (plywood) is calculating a higher cubic ftg number than I was physically calculating. I believe that this is due to an offset in the computer system, but I wanted to make sure that I was using the most accurate formula before I submitted my report. I really appreciate your time, and apologize for not finding the formula before writing. I was not aware that the situation or example is termed a "frustum". I was looking in other areas. Wonderful Job!!
Date: 08/08/2002 at 12:09:59 From: Doctor Peterson Subject: Re: Thank you (Cubic Footage of a Tapered Log) Hi, Christopher. I was intrigued by the formulas I found researching your question, because I found information about formulas for the parabolic frustum only in forestry sites, which didn't make it clear whether they are exact or approximations (since they seem so different). Our site doesn't even list the formula for volume of a segment of a paraboloid, V = pi r^2 h/2 Looking a little deeper than I did last night, I found that "Smalian's formula" is correct for a parabolic frustum V = pi (r1^2 + r2^2) h/2 or V = h(A1 + A2)/2 (where h is the height, r1 and r2 are the bottom and top radii, and A1 and A2 are the bottom and top cross-section areas), while "Huber's formula" V = pi rm^2 h or V = h Am (where rm and Am are the radius and area of the section halfway between the top and bottom) is also correct, because for a paraboloid rm^2 = (r1^2 + r2^2)/2 Further, "Newton's formula" V = pi (r1^2 + 4rm^2 + r2^2) h/6 or V = h(A1 + 4Am + A2)/6 is valid for both the parabolic frustum, the conical frustum, and in fact any shape (called a general prismatoid) whose cross-sectional area follows a linear, quadratic, or cubic law, which also includes spherical segments and more: http://mathworld.wolfram.com/GeneralPrismatoid.html All this is beyond what you need, of course; I just wanted to fill in some gaps in my list of formulas. If you need any further help figuring out where the errors are arising, feel free to send me additional information. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/
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