The following explains how the Texas Tree Trails measures champion, historic, and significant trees in the field. This guide covers the standards, formulas, and step-by-step methods used by our volunteers — based on the criteria set by American Forests and the Texas A&M Forest Service.
How to Measure a Champion or Big Tree
The Standard
The forestry and tree-management industries use established measurement methods to produce results that all parties can agree on. Field estimation techniques have their inaccuracies, but the standard error for widely-used methods is fairly similar — comparing the stick method of height measurement against a small handheld clinometer, for example, yields very small differences in standard error.
For most practical purposes, simple manual field techniques produce equally acceptable results as more sophisticated methods, within the limits of error. Texas Tree Trails uses the simpler field methods, following the rules and guidelines set by American Forests and the Texas A&M Forest Service.
When indirect methods are used to measure height (as opposed to direct methods like climbing or telescoping poles), readings should be taken from two independent positions, preferably on flat ground. The two readings should agree within the limits of measurement error. This is an absolute check on instrument and operator error. Differences of up to one meter for a 40-meter tree are acceptable — precision with hand-held instruments under forest conditions is no better than this.
The Tree Index Formula
Three measurements are required to evaluate a tree for inclusion in the National, State, or DFW Big Tree Registries:
- Trunk Circumference, in inches
- Total Height, in feet
- Average Crown Spread, in feet
The tree’s index — a normalized comparison factor with no units other than points — is the sum of the circumference in inches, the height in feet, and one-quarter of the average crown spread in feet:
Circumference + Height + ¼ Average Crown Spread = Index Points
Trees within five points of each other are considered co-champions.
Recorded Values
All measurements are rounded down to the nearest whole number. So, 48.9 feet is recorded as 48 feet; 132.6 inches as 132 inches.
The Ten-Year Rule
Trees must be measured again at least every ten years to maintain their champion status.
What Is a Tree?
Each specimen nominated for the Texas Big Tree Registry must meet this definition: “Trees are woody plants, having one erect perennial stem or trunk at least three inches in diameter at breast height (DBH, or 4.5 feet), a more or less definitely formed crown of foliage, and a height of at least 13 feet” (Little, 1979).
For low-forking specimens, this means that one of the forks must exceed 9½ inches in circumference at 4.5 feet to qualify (see diagram A).
One Tree or Two?
You’ll often need to decide whether a tree has a single trunk or whether you’re looking at two or more stems growing very close to one another. Some rules:
- Trunks with clear separation, or with included bark at or near the ground line, should be considered separate trees.
- Trunks of different species are always separate stems, no matter how close together.
- If following the circumference rules below places the measurement point at the ground line, the stems should be considered separate.
Circumference
General rule: Record the smallest trunk circumference between the DBH point (4.5 feet) and the ground, below the lowest fork. Also record the height above the ground (in inches) where you took the measurement (diagrams B and C).
Determining DBH oF a Tree on a Slope
Measure up 4.5 feet along the axis of the trunk on both the high and low sides; the DBH point is midway between these two planes (D).
Determining DBH on a Leaning Tree
Measure 4.5 feet along both the top and underside of the trunk; the DBH point is midway between these two planes (E).
Low Branches
When determining where on the trunk to measure circumference, ignore portions that aren’t part of the tree’s crown, such as dead branches, forks, or epicormic sprouts (sprouts that come from dormant buds beneath the bark).
Obstruction at DBH
If there’s a bump, burl, branch, or other obstruction at the DBH point, measure circumference above and below the obstruction and record the smaller value. A buttress that forms naturally between the trunk and root system as a feature of the species — for example in bald cypress or water tupelo — is not an obstruction and shouldn’t be treated as one.
Tree Height
General rule: Record the vertical distance between the ground line and the tallest part of the live crown, in feet. Also record the method used.
Common methods used include direct measurement (telescoping pole or climbing), clinometer, hypsometer, relascope, ultrasonic or laser rangefinder (with or without internal clinometer), stick method, pencil method, comparison, and — yes — informed guess.
Leaning trees: Height is measured to the tallest point of the live crown, not along the length of the trunk.
Stick Method (recommended for beginners)
The simplest field technique. You need a ruler or pencil, good eyesight, and a friend.
One person (Person 1) stands near the trunk of the tree. The second person (Person 2) stands at a distance where they can see both Person 1 and the top of the tree clearly.
Person 2 holds a ruler upright at arm’s length and walks forward or backward until the ruler exactly covers the tree from base to top (F). Keeping the ruler at arm’s length, Person 2 then turns their wrist, so the ruler is horizontal, with one end sighting the base of the tree.
Person 2 then instructs Person 1 to walk away from the trunk (at a 90-degree angle to the line of sight) until Person 1 is standing where the end of the horizontal ruler now points.
Person 1 is now standing roughly the same distance from the trunk as the tree is tall. Use a tape measure to record that distance, in feet.
This works because of similar triangles:
B = A × (b / a)
Where A is calculated from the Pythagorean theorem (A = the square root of [technician’s height² + distance to tree base²]).
Pencil Method
A close cousin of the stick method. Hold a pencil or pen at arm’s length and move toward or away from the tree until the top of the pencil lines up with the tree’s highest point and the bottom of the pencil lines up with the spot directly below the highest point.
Then rotate your hand so the pencil is “laid down” horizontally on the ground, keeping the bottom of the pencil at the same point. Mark the spot where the top of the pencil now points to the ground (a second person helps here).
The tree’s height equals the distance from the pencil-bottom point to the new pencil-top point — provided that point is at a right angle to your line of sight.
Using a Clinometer
A clinometer is a simple mechanical instrument for measuring tree height by sighting and applying trigonometry to measured angles and known distances. Steps for the Suunto Clinometer:
- Measure the horizontal distance from the base of the tree (or the position directly beneath the tip of a leaning tree) to a location where the required point on the tree is visible.
- Sight the required point (usually the tree tip):
- One eye: Close one eye and look through the clinometer at the scale while simultaneously looking past the clinometer at the tree. Judge where the horizontal line on the scale would cross the tree.
- Both eyes: Look at the scale with one eye and at the tree with the other; let the two images appear superimposed and read where the horizontal line crosses the tree. If you have astigmatism, use the one-eye approach.
- Read the percent scale, then multiply that percentage by the horizontal distance from step 1.
- Sight to the base of the tree and repeat the percent-and-multiply step.
- Combine the two heights:
- Add them if you looked up to the tip and down to the base.
- Subtract the base height from the tip height if you’re on sloping ground and had to look up to both.
- Check all readings and calculations.
Simple Ratios Method
For some, this is the easiest method, but it requires two people: the measurer and the “measuree.” Measure the measuree’s actual height and record it in feet and inches (say, 5 feet 6 inches – or 5 1/2 feet, or 5.5 feet).
The measuree stands directly beneath the highest point of the tree. The measurer holds a yardstick perpendicular to their body, like in the stick method, and sights the measuree’s full height between two convenient marks on the yardstick — say, between 5 and 8 inches. That means the measuree’s height equals 3 inches on the yardstick at that distance from the tree.
Without moving their arm, the measurer carefully tilts their head back and sights the topmost point of the tree, reading where it falls on the yardstick — say, 27 inches. That means the tree appears to be (27 − 5) = 22 inches on the yardstick at that same distance.
Use a ratio formula:
(measuree height / 3) = (tree height / 22)
5.5 / 3 = x / 22
x = 40.3 feet
Average Crown Spread
General rule: Along the drip line of the tree (H), take two crown-width measurements at right angles to each other, in feet. The first is the widest crown spread (I): the greatest distance between any two points along the drip line. Once you’ve found the widest spread, turn the axis of measurement 90 degrees and find the widest crown spread in that plane (J). Average the two:
Average Crown Spread = (Major + Minor) / 2
The drip line is the outline on the ground of the outermost leaves of the crown (H). Only live portions of the crown count.
Tools and Skills Required
Recommended Tools
Required: yardstick or meter stick · measuring tapes (see your regional forester for specifics) · clipboard and forms · writing implements · camera
Recommended: cell phone · portable GPS
Optional: clinometer
Safety Equipment
Hard hat, work gloves, work shoes, and coveralls when working in the field.
Recommended Skills
Master Naturalist or Master Gardener training is helpful, as is college-level coursework in silvics, botany, or similar fields. You should be comfortable with basic geometry and simple algebra, comfortable interacting with the public, physically able to walk over varied outdoor terrain, and willing to take ownership of your part of the work and explain it to others.
Training Sessions
Classroom instruction is offered periodically by local state foresters and regional certified arborists. Sessions include a classroom lecture covering basic tree identification, regional tree knowledge, and hands-on field measurement techniques, followed by a field trip to a regional park where trainees measure actual trees.
Registration is required. To find out about upcoming sessions, email info@texastreetrails.com.
Data Collection
Texas Tree Trails uses the standard data set established by the Texas A&M Forest Service. Every tree record requires a quality set of photographs. At minimum, one vertical photo showing the entire tree in view, plus a series of additional horizontal shots highlighting leaves, fruit, bark, form, flowers, structural features, damage, roots, and trunk.
Notes, sketches, botanical observations, observations on the tree’s health, evidence of damage, potential hazards, placement relative to surroundings, soil, and wildlife use are all helpful additions to the record.