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3-9-17

Measuring tree diameter

After recording tree species, the next step is to measure the diameter of each tree. Diameter is the width of a circle or cylinder; in this case the stem of the tree. Tree stems have taper (they are slightly cone-shaped), such that they are wider at the base and narrower further up. Thus, diameter will vary based on how high up on the stem of the tree you measure it.

A standardized height for measuring tree diameter has been established. This is known as breast height, which is defined as 4.5 feet (54 inches) above the ground, on the uphill side of the tree.

Use a measuring tape to figure out how high up breast height is on you. Memorize this spot (e.g. the next to the top button on your shirt or coat so that as you stand next to a tree, you know where to measure diameter.

Using a diameter tape

We will talk about two ways to measure diameter at breast height (DBH). The most accurate way is to use a special measuring tape known as a diameter tape. A diameter tape is calibrated such that circumference measurements are automatically converted to diameter (i.e. the measurement units have been divided by the constant Pi so that you do not have to do the mathematical conversion from circumference to diameter). Thus while you are physically measuring the circumference of the tree, you are reading the measurement in diameter units. If you do not have a diameter tape, you can use a regular cloth measuring tape and divide your circumference measurement by Pi (3.14) to convert to diameter.

To measure DBH, wrap the diameter tape around the tree at breast height, making sure the tape is level and not twisted. Most diameter tapes will give you diameter to the nearest 1/10th inch. The diameter measurement is read where the tape overlaps with the zero marker (Figure 6-1). If the tree is large, it is helpful to work with a partner to wrap the tape around the tree. Many diameter tapes also have a nail or hook on the end that you can stick into the tree to hold the end of the tape in place while you wrap it around.

Using a clinometer

As with DBH, we will talk about two different ways to measure height. The first way is to use a tool called a clinometer. A clinometer is a vertical angle gauge that measures the slope from your eye to either the top or bottom of the tree. Looking with one eye into the hole of the clinometer, you will see a graduated scale showing the slope measurements. The scale will move up or down as the clinometer is tilted up or down. Different clinometers measure slopes in different units, so it is important to consult the instructions for your clinometer to confirm what units are being measured. Commonly clinometers list the slope in degrees on the left half of the scale and in percent on the right half of the scale. The steps described below correspond to percent slope measurements. By definition, percent slope is the distance up or down per 100 feet of horizontal distance. Thus a reading of 30% means a height or depression of 30 feet, when measured at 100 feet horizontal distance.

When measuring the height of a tree, you will need to move away from the tree to a place where you can see the top. While you can choose any distance at which you can see the top of the tree and the clinometer reading is not off the scale (usually 150%, though accuracy may deteriorate beyond 120% or so), because you are working in percent a distance of exactly 100 feet will greatly simplify the calculation to determine tree height. For trees that are tall enough such that 100 feet is not far enough back to accommodate the scale of the clinometer (or to see the top of the tree), you will have to move back further to obtain an accurate reading. For very short trees, you may only need to move back 50 or 75 feet.

Once you have measured the distance from the tree, you can use the clinometer to measure the slope between your eye and both the top and bottom of the tree (Figure 6-3). Looking into the clinometer with one eye (you cannot actually look through the clinometer) and sighting the top of the tree with your other eye, visually line up the horizontal marker in the clinometer with the top of the tree (Figure 6-4). It may take some practice to get your eyes to work together for this. Once the horizontal marker in the clinometer is lined up, read the corresponding value on the percent scale. Now repeat this procedure for the bottom of the tree.

Assuming the angle to the top of the tree reads a positive value, if the angle to the bottom of the tree reads a negative value, you add the bottom value to the top value to get the total percent slope. In the case where the angle to the bottom of the tree also reads a positive value, such as when you are standing downhill from the tree (Figure 6-5), you would instead subtract the bottom value from the top value to get the total percent slope.

1. What is diameter in reference to trees?

2. How high off the ground is "breast height?"

3.  How is the measurements on a diameter tape calibrated?

4.  What does DBH stand for?

5.  What is a clinometer?

6.  How do you look through a clinometer?

7.  Which side of the center line do you read the tree's height?

8.  List the steps of using a clinometer correctly?