Once you have determined at what height you will be measuring each tree you will use one of several tools to measure the diameter.
A Diameter tape (D-Tape) is frequently used to accurately measure the diameter of a tree. A D-Tape is a measuring tape that has different units of measure on each side. One side of the D-Tape is marked off in either inches or centimeters and the other side is marked off in the same unit multiplied by 3.14(pi). When measuring diameter have the second side facing out and wrap the D-Tape around the tree. Line up the 0 mark with the end of the tape and record the number shown. When the 0 mark does not end up with a full number be sure to count up from the smaller of the two numbers it is between and not the larger. The end measure that you get will be the diameter of the tree, not the circumference as you would expect from a normal tape measure. Diameter measurements should be taken around the tree, perpendicular to the direction of growth. When using a D-Tape be sure to pull all of the slack out of the tape so it is tight against the bark of the tree.
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A Blitmore stick is another tool that can be used for measuring the diameter of a tree. In overall appearance a Biltmore stick looks a lot like a standard yard stick but it has unique markings that foresters use to determine height and diameter. To determine diameter with a Biltmore stick the practitioner holds the stick against the side of a tree at 4.5 feet and a prescribed distance from their eye, the standard Biltmore stick is calibrated for 25". Then by aligning the left side of the stick with the left side of the tree they read off the number on the right side of the stick that lines up with the right side of the tree. This number is less accurate than using a D-Tape, but for a basic inventory it is adequate.
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Calipers are another option that can be used to measure a tree's diameter. While calipers are generally used on smaller trees, and most nurseries sell their smaller trees as a specific caliper, they are also made for larger trees as well. In order to use a set of calipers you simply expand the jaws large enough to allow the tree to fit between them, then slide them closed until both sides are snug against the sides of the tree. In order to get an accurate measurement be sure to hold the calipers perpendicular to the stem of the tree. When using calipers two measurements should be taken perpendicular to each other and averaged to account for any nonconformities in the trunk.
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