Wednesday, January 21, 2015

Volume Calculations and Rules, Part 1: Log Rules in Overview

Over the next few posts, we’ll be diving into the wonderful world of log volume calculations. In this first post, we’ll provide a brief explanation of the four main log rules, or methods for determining log volume, as well as the primary strengths, drawbacks, and uses of each method. We know it can be hard to keep track of it all, so we’ve tried to simplify and summarize them in a way that makes sense.

Our hope is that this information will help you to be better informed when presenting the results of a cruise to a client, selecting a log rule for your next project, or even just getting to know your field better!

Tons and cubic feet
The first method is to estimate the volume based on weight, which is measured in tons. This method is usually employed when trying to determine the volume for pulpwood purposes, though used for sawtimber in some regions. It can also be useful for carbon accounting or other, non-lumber calculations. In this method, the tree or log is measured for length and diameter. These numbers are plugged into an equation (some treat the log as a cylinder, some as a cone, and some a combination of both), which yields an approximation of volume in the units of tons or cubic feet.

The next three log rules share a common volume unit, board feet. You probably remember that 1 board foot is equivalent to a 1”x12”x12” board. This makes it really easy to compare results from the different calculation methods, as the output units are the same.  These rules were originally developed to estimate mill output from standing timber or logs, and were used to guide sales.  Most were developed regionally, for specific mill and forest conditions, and continue to be used today despite advances in milling technology and more precise inventory methods.

The Doyle log rule is widely used in many parts of the country because of its simplicity and versatility. It uses an equation to calculate board feet based on the length of the log and its diameter at the small end. The equation does not account for the taper of the log or saw kerf. Because of this, and the nature of the equation, volume estimates tend to be too low for small logs and too high for larger logs. However, the effect will basically cancel itself out in a stand of diverse age and size, so the total volume estimate will be fairly accurate.

Scribner Decimal C
The Scribner log rule works best for conifers and other tall, straight trees, so it is most commonly used in the Western United States. It’s very easy to use, because it doesn’t take things like the taper or species of the tree into consideration. It only needs the length and diameter of the tree, which corresponds to a point on the table that shows the approximate volume. Behind the scenes, the Scribner Decimal C log rule is a variation of the Scribner rule that rounds to the nearest 10 board feet. It also compensates for a ¼” saw kerf in the final product.

Photo credit: Charlie Houder.
International ¼”
The International ¼” log rule is the most accurate rule out there. Like the Doyle log rule, it uses geometric equations to determine the volume. The reason it’s so much more accurate is that it calculates the volume based on four-foot sections, while also accounting for the ¼” kerf. This significantly reduces the errors caused by tree taper, which can crop up in the other methods. 

Variations of this rule allow volume calculations for sections of differing lengths, but it can be very time-consuming to make all those calculations without an efficient, automated system.

It can be very challenging to determine which log rule is right for your project. Sometimes, regional variations or tree type will make it clear which rule is preferable, and sometimes the organization you are working for will demand a certain rule to be used, so it’s important to be familiar with all of them.

The good news is, here at SilviaTerra, we can do them all! We’ve specially designed our volume system to be 100% compatible with each of these main log rules, which makes your life, and your organization’s work, that much easier.

For more information, we recommend visiting the Dictionary of Forestry from the Society of American Foresters. They go into a lot of depth and get down into the nitty-gritty of the terms and definitions we’ve used here. The information in this post was drawn from Forest Measurements by Avery and Burkhart, and the aforementioned Dictionary of Forestry. We also recommend you check out this resource from Purdue University if you want to study log rules and log scaling in more detail.

Now that we’ve got the basics of log rules down, be sure to check back soon for our next post in the series, where we will dig into individual tree volume calculation methods and begin to discuss form classes.