TERRA: Champion Trees

Onondaga Lake Peace Festival

Measuring a Candidate Tree

Bob Leverett
sights the highest branch
with his laser rangefinder

Currently, a Champion Tree is simply the biggest. The sole criteria to rate a champion is simply size.
But how to evaluate size? With over 900 tree species in America, can a single, simple method measure and rank all of them to determine which is biggest? The smallest is only 11 feet high, while the tallest is 321 feet. Some—like spruces and redwoods—grow tall and thin; others are short with stout stems—like oaks. Some species have wide-spreading canopies; others are tall with few limbs—like a giant toothbrush.
The national standard to evaluate trees to determine which is biggest employs a formula based on three measurements:
circumference
height
crown spread

But even these three measurements are not simple, or easy to take, given the great variations in shape and proportion of trees and species. Often, the sheer size of these giants presents a serious challenge to someone with a simple tape rule. Even professional big tree hunters debate the most accurate ways to take these measurements. Measuring big trees can be so tricky, some states require all identification and measurements to be verified by a qualified professional.
To help you take reliable, accurate measurements, the Champion Tree Project has drafted guidelines detailing the steps of measuring a big tree. These instructions are based on acceptable international standards to scientifically record the dimensions of trees. Print this page, along with the Nomination Form and the guidelines to "Nominate a Big Tree," and take them with you to the tree. We hope this information answers all your questions and suffices to cover all types of circumstances you may encounter. But if you have any doubts or questions, or discover a tree with an unusual and confounding problem, please email us.
Girth
Girth is measured as the circumference in inches at breast height, which is four and one half (4.5) feet above ground level (1.37 meters).
Girth the easiest dimension to measure. In most situations, this measurement can be taken with a simple tape rule, although many champions require a tape rule 25 feet long, and a few even need a 50 footer. For very large diameter trees, care must be taken to assure the tape rule doesn't sag below 4.5 feet as it snakes around the trunk.
This simple procedure suffices for most trees growing on level ground, but several unusual circumstances may be encountered. Trees may be growing on ground that isn't level, or even steeply sloped. Or a tree may have branches below 4.5 feet, or have multiple stems. The only time a tree can be measured below 4.5 feet is if that measurement is less than the dimension at 4.5 feet.

Measuring on Slopes: When the ground around the trunk isn't level, measure 4.5 feet above the lowest point where the trunk meets the ground, then measure 4.5 feet above the highest contact of tree with earth. Then take the average of these two distances, or halfway between these two points on the trunk.
Measuring on Very Steep Slopes: On occasion, the slope is so steep that the average point is lower than the highest point where the trunk touches the ground, making a girth measure at this point impossible. In such a case, measure the girth at the lowest practical point, which usually means at the highest point where the trunk meets the ground.
Measuring a Leaning Tree: When a tree is leaning significantly away from vertical, measurement should be made at right angles to the axis of the trunk, not in the usual horizontal plane. The measurement should still be taken at a point an average of 4.5 feet above ground level, measured vertically, not along the leaning trunk.
Measuring a Low-Branching Tree: Some trees have a branch or growth projecting from the trunk below 4.5 feet which obstructs or influences the girth measurement. In such a case, circumference is then measured at the point below breast height which yields the smallest measurement.
Measuring a Multi-stem Tree: A few species often develop sprouts that form multiple stems at 4.5 feet. In these instances, the largest stem should be measured at 4.5 feet.
Measuring a Tree with Butt Swell: A tree—particularly a few tropical species such as baldcypress—may have a swollen circumference near ground level which rapidly tapers to a much narrower trunk. Measurement must be made at 4.5 feet, although a second circumference measurement can be taken at a higher level that is useful to convey a clear description of trunk size.

Height
Height is measured as the distance in feet between the base of the tree's trunk and the topmost twig.
Height is usually much more difficult to measure than girth, especially given the extra-ordinary size of most champion trees. Obviously it's not feasible to climb a tree and drop a tape measure from the tip of the highest branch. Precise measurements require the use of a sophisticated instrument, such as a transit, hypsometer or relascope. Hand-held devices like an Abney level or clinometer can yield accurate data if used carefully. In the absence of such a measuring tool, a rough estimate can be obtained by a simple manual sighting technique using a straight ruler, tape measure and a little trigonometry, as described below.
The first challenge is to find a spot which permits an unobstructed view of both the base and the top of the tree. Ideally this view should be over flat, level ground, but this is rarely the case. For simplicity, the instructions for the two methods detailed below assume the tree can be sighted on level ground.
Estimating Height by Manual Sighting
Method 1: Hold a ruler of fixed length (one, two or three feet) at an arm's length in front of you, and step backwards from the tree until the line of sight through bottom of the ruler touches the tree's base, and the line of sight over the top of the ruler touches the topmost twig of the tree. Ideally, this sighting should be taken with your eyes as close to the ground as possible to assure the sighting triangle is a right triangle.
Measure the distance (in feet) from your eyes (O at right) to the ruler (B at right), and from your eyes to the base of the tree (C at right). The height of the tree is then distance C multiplied by distance B, and divided by distance A. For simplicity, if the ruler B is one foot, then the height of the tree is simply C divided by A.
Method 2: Make a clearly visible mark on the tree trunk at a fixed, measured height above ground level—say, four feet, or six feet. Hold a ruler in front of you and step backwards until the one inch measure on the ruler matches the distance from the ground to the mark on the trunk. Notice how many inches on the ruler match the entire height of the tree. The height of the tree is then the number of inches multiplied by the height in feet of the mark.

Bob Leverett
sights a big tree with his clinometer

A tree whose height is estimated by these manual sighting methods should be remeasured by a qualified person with a reliable instrument to verify its height before being added to a big tree list.
There are several exceptional circumstances which can further complicate the accurate measurement of height. In fact, most trees present at least one of these exceptional challenges.
Measuring a Tree on Sloping Ground: Very frequently, a tree isn't growing on level land, but on a significant slope. In this case, measure the highest and lowest points where the trunk base meets the ground, and measure from the average point between these two. Conduct the height measurement as if a horizontal plane is centered on this midpoint on the base of the trunk.
Further, the sloping terrain complicates the simple trigonometry detailed above. One solution to this dilemma is to mesure the compound angles from eye level up to the topmost limb, and from eye level down to the base of the tree trunk. Taking these measurements requires sophisticated equipment, and making the calculations is beyond the scope of these simple instructions.
Measuring a Leaning Tree: A leaning tree is especially difficult to measure, especially the further the tree deviates from vertical. A slight lean can be ignored, if it means only a small variance in calculated height. However, strong leans require a more complex calculation to determine the distance from trunk base to top of crown along the axis of the trunk. The complex techniques and formulas required for these difficult measurements are available on special request. An untrained person is advised to seek assistance from a skilled professional.

Average Crown Spread
Crown Spread is measured as the distance between the outermost tips of the farthest reaching limbs of the canopy. It is measured along the widest and the narrowest axes, and then the average of these two maximum and minimum dimensions is computed.
Average crown spread often can also be a difficult measurement to take accurately. Two measuremnts must be made: first of the widest spread of the tree's limbs, then the shortest spread of branches. Only living branches should be measured; dead limbs should be ignored. To do this, stand under the farthest reaching branch, then measure to the farthest reaching branch on the opposite side. Repeat this procedure with the two limbs with the narrowest width.
Ideally, this would be done by dropping a plumb bob from the end reach of each limb, but, since this is impossible, you will have to rely on careful eyeball estimates. For truely big trees, these measurements are easiest to make with two people and a 100 foot tape measure.
Average crown spread is computed by adding the two measurements together, then dividing by two. In particularly difficult circumstances, mark the outline of the crown with stakes or stones and take several measurements to determine which axes are the wideset and narrowest.
As before, certain conditions can pose further complications to taking accurate measurements:
Measuring on Steep Slopes: Ideally, crown spread should be measured on level, horizontal ground. When measurements taken on a slope are stretched, and have inches and feet added to them. The steeper the slope, the greater the error. Try to take measurements as much on a level plane as possible.
Measuring a Low-Branching Tree: Trees with a branch or growth on the trunk below 4.5 feet should have the entire crown included in the measure, including the low-branching limbs.

Total Points
Champion Trees are scored by points. Total Points are computed by a simple formula based on these three measurements. Total Points is computed as girth in inches, plus Height in feet, plus one quarter the Average Crown Spread in feet. This formula is weighted heavily in favor of girth, while crown spread is weighted least.
Volume
On occasion, a controversy arises between two or more trees for Champion standing. Under certain conditions, these disputes can be resolved by volume measurement. Some argue that volume is really the final word in big tree status, since it combines girth and height in a single measurement that is the total amount of wood enclosed by the tree. Many of the largest trees (Sitka Spruce, Redwood, Sequoia, Douglas Fir) have volume data routinely included with other data.
While volume of a single tree may be of value, when ranking big trees, measurements are only valid if applied to two or more trees measured by the same persons using the same method or instrument. Generally, there is agrement that if ranking by Total Points of the standard formula differs from ranking by volume, the volume ranking determines the champion. This is because volume represents a more complete measure of tree size.
Volume measurements can only be made using a sophisticated instrument such as a survey laser, using accepted forestry measuring techniques. The bulk of the tree must be a tapering, near-cylindrical trunk; trees with massive spreading branches are inappropriate for volume measurements. The trunk must be measured at regular intervals and reduced to a series of cylinders. This is beyond the scope of the simple methods provided here.