Diameter Measurement
Tree diameter is the most important measurement of standing trees. The point of measurement is 4 1/2 feet above the ground line , a point referred to as "diameter breast height" or "DBH." Diameters are usually measured to the nearest inch, but where large numbers of trees are to be scaled, 2-inch diameter classes are used.
Photo 1 - Determining your
'reach' is easy using a yardstick.
Place the zero-end of the
yardstick at your eye and fully
extend your arm. Read the
measurement at the end of
your thumb.
The tree scale stick used to measure the tree's diameter and its height needs to be calibrated for the length of your reach. Determining your "full arm" reach is simple. All you will need is a yardstick. Have a partner stand to either side of your body. Put the zero-end of the yardstick up to your eye and full extend your arm along the yardstick. The distance between your eye and the end of your thumb is your "full arm" reach (photo 1). Have your partner read this distance off on the yardstick. Once you know this distance, you can select the appropriate length tree scale stick for your reach. There are three Invitational tree scale sticks that can be downloaded from the Invitational web site; a 20-inch reach stick, a 23-inch reach stick, and a 25-inch reach stick. If your "full arm" reach is between 20-23 inches, select the 20-inch reach stick. If your "full-arm" reach is between 23-25 inches, select the 23-inch reach stick. If your "full arm" reach is 25 inches or greater, select the 25-inch reach stick.
Tree diameters are measured at 4 1/2 feet above the ground, a point referred to as diameter breast height (DBH). Determine where the 4 1/2 foot mark is on your chest/neck area. You can use the yardstick to determine this measurement.
Assuming you have selected the appropriate tree scale stick for your reach, stand facing the tree's trunk and position your body so it is in the center of the trunk. Hold the tree scale stick horizontally against the tree at 4 1/2 feet above the ground line and push yourself back away from the tree to the arm's length reach that your scale stick is calibrated for; 20-, 23- or 25-inches (photo 2 and 2a). It is important that distance between your eye and the scale stick be 20, 23 or 25 inches depending on the stick's calibration. If this distance is less or greater than what the stick is calibrated for, the diameter measurement you determine will be wrong.
Photo 2 and 2a - To measure the tree's diameter, position yourself in the center of the trunk. Hold the scale stick against the tree at 4 1/2 feet above the ground line and push yourself back away from the tree the required distance that your scale stick is calibrated for; 20 inches, 23 inches or 25 inches.
Make sure you hold the scale stick firmly against the bark of the tree. Now shift the stick right or left until the zero end of the stick is flush with the left edge of the tree trunk. Without moving your head (move only your eyes, not your head), look over to the right edge of the tree's trunk and read the diameter measurement on the scale stick where the right edge of the tree trunk intercepts the stick (photo 3). Diameter of standing trees is measured outside the bark.
Photo 3 - At the 4 1/2 foot height, move the tree scale stick right or left to line the zero-end of the scale stick up with the left side of the tree trunk. Without moving your head (move just your eyes), look over to the right side of the trunk and notice the diameter reading where the right side of the tree intercepts the scale stick (yellow arrow). If you are holding the scale stick the correct distance from your eye, this reading is the tree's diameter. Diameter should always be measured on the uphill side if the tree is growing on sloping ground.
Because a tree's trunk is not round and is often more egg-shaped, it is necessary to take two diameter measurements to get an average diameter reading. Take the second diameter measurement 90 degrees to the first measurement (photo 4). Add the two diameter measurements together and divide by 2 to determine the tree's average diameter. It is important to remember that on sloping ground always measure diameter on the uphill side of the tree.
Photo 4 - Tree's are rarely round, so you need to take two diameter measurements at 90 degrees to each other. Add the two measurements together and divide this total by 2. The result will be the average diameter of the tree.
