What you don’t know that you don’t know?

Spring 2015

Forestry, like any other profession, contains certain terms or practices that become so commonly used that their origin is sometimes forgotten. There is a constant need to measure various aspects of the forest environment including distance and area. You may have heard of a linear measure called a chain, or of something called the Scribner Decimal C Log Rule. This issue of Tree Talk will explore these two archaic terms that are used every day in forestry.

THE GUNTER’S CHAIN
Forestry, like any other profession, contains certain terms or practices that become so commonly used that their origin is sometimes forgotten. There is a constant need to measure various aspects of the forest environment including distance and area. You may have heard of a linear measure called a chain, or of something called the Scribner Decimal C Log Rule. This article will explore these two archaic terms that are used every day in forestry.

I once mentioned to my brother-in-law that the term “chain” is a common unit of measurement. A U.S. Military Academy graduate, he immediately replied: “My civil engineering professor at West Point told us about the chain. It’s an archaic system last used 200 years ago.” I begged his pardon and explained that many foresters use the “chain” as a basic measurement of distance. We look at an area of ground, a planting unit for instance, and estimate its rough dimensions in chains to quickly calculate its acreage. For example, if I estimate an area as 5 chains by 10 chains, I immediately know that it is about 5 acres. (5 x 10 = 50 square chains ÷ 10 square chains per acre = 5 acres).

So what is a chain? How did it come about? What does it mean?

Edmund Gunter of Wales (1581-1626) invented the chain as a measuring device. Both a clergyman and mathematician, he served as rector of a parish church in London as well as an astronomy professor at Gresham’s College. But Gunter’s real passion seemed to be developing instruments to apply newly applied trigonometric and geometric principles to facilitate surveying and earth measurements.

In Gunter’s era, the English measured distances in miles, feet and inches, and land area in acres. An acre at that time was defined as the amount of agricultural land that could be worked by one man in one day. Initially an acre varied in size because rocky ground on a slope took longer to work than fertile, flat terrain. But, over time, the dimensions for an acre were agreed upon as four square perches. A perch (also called a rod) is 16.5 feet in length. Four perches equaled 66 feet, 320 perches equaled one mile (5,280 feet), and a square mile contained 640 acres. A 16.5- foot staff (or rod) was used to measure distances and plot acres.

About 1607, Gunter developed a measuring instrument he described as: “For plotting of ground, I hold it fit to use a chain of four perches in length, divided into an hundred links.” Since Gunter used metal because of its durability and resistance to shrinkage or stretch, and since each of the 100 links oddly measured 7.92 inches “four perches” totaled 66 feet which became known as one chain.
Gunter’s new measuring device immediately found useful applications. For instance, 80 chains equaled one mile and 10 square chains equaled one acre.
Fast forward a couple of centuries from Gunter’s era to when a young nation, called the United States of America, sought a system to survey its expanding western borders. Congress eventually adopted a model for public land surveys comprising townships, ranges and sections. The basic one mile square unit, called a section, totaled 640 acres. The field survey technology of the day consisted of a Gunter’s chain and staff compass. Surveyors measured 80 chains between corners of each section. As homesteading began in the western territories, sections were each typically subdivided into quarter sections of 160 acres that measured 40 chains by 40 chains. Even further subdivisions broke sections into “40s” or 40 acre units of 20 chains per side. Gunter’s chain made the mathematics simple for this survey system. The terrain, however, provided the challenges for the surveyors with their rudimentary instruments.

Modern technology – GPS and GIS – is quickly pushing the Gunter’s chain as a measuring unit into obscurity. Instead of estimating the dimensions of a planting unit, it’s just as easy to walk around its perimeter with a GPS in hand and return to the office to upload the data directly onto a GIS map. Easier yet, just bring up the latest aerial photo on GIS and use the computer mouse (rather than boots on the ground) to traverse the planting unit and know very accurately its size and location. But that’s no fun; foresters still want to go to the woods. If only I could get the GPS and GIS makers to include the “chain” as one of their scale units!

(By the way, the length of an English Cricket pitch is 22 yards or ONE CHAIN. Tell an Englishman that’s archaic!)

SCRIBNER DECIMAL C LOG RULE

Log scalers measure logs to determine the quantity of usable lumber they contain. Lumber is a square or rectangular product sawn from a more or less round log. Both are measured in a unit called a board foot. A board foot is described as a piece of wood that measures one inch thick, 12 inches wide, and 12 inches long.

More than 100 systems, called log rules, have been developed to estimate the board foot content, or volume, contained in a round log. While it is easy to calculate the volume of a cylinder, a log is never a perfect cylinder. Factors such as taper, sweep or crook, and defect present challenges in developing log rules that accurately estimate board foot volume. So log rules are developed using diagrams, mathematical formulas or mill tally. The Scribner Log Rule is based upon diagrams while the other two other most commonly used log scales – the Doyle Log Rule and the International Log Rule – use formulas to derive the board foot volume of a log.

In the Inland Northwest, we use the Scribner Decimal C Log Rule. Here’s how it came about.

An Internet search reveals little about the life of J. M. Scribner. Another man of the cloth, clergyman Scribner apparently didn’t hold religion as his only interest since in 1846 he developed a log rule to estimate the volume of lumber that can be sawn from logs.

The Scribner Log Rule uses a series of scale drawings depicting the number of one inch boards using a ¼ inch saw kerf that could theoretically be sawn from logs ranging in size from 12-44 inches inside of the bark at the small end of the log. Unfortunately, Scribner’s drawings did not account for taper in the log. The Scribner Log Rule is reasonably accurate for larger logs without much taper, but it under scales small diameter and long logs, which seem to be the preference of sawmills today.

Over time, a modification of the Scribner Log Rule began rounding to the nearest 10 board feet, which resulted in all numbers ending in a zero. Then, the decimal point was shifted one digit to the left thus dropping the zero. For example, a log that actually scaled 142 board feet was rounded to 140 board feet, but written as 14 Scribner decimal volume. This made it much easier to add long columns of log volumes and the rounding helped to balance out the inherent inaccuracies over a number of logs. Other modifications of the original log rule were identified as A, B, or C. We now use the C version, resulting in the Scribner Decimal C Log Rule.

Custom and tradition keep the Scribner Decimal C Log Rule in common use despite its inherent inaccuracies. Everyone grumbles about it, but 161 years later it still provides the basis for determining the volume, and hence, the value of logs delivered to a sawmill in the 21st Century.

Conclusion
No doubt you have heard the terms chain and Scribner Decimal C Log Rule. No longer can you say “I don’t know what I don’t know” about these commonly used terms.