A diameter tape (D-tape) is a form of dendrometer that consists of a cloth or metal tape that is mainly used to measure diameter at breast height (DBH). DBH is measured at a fixed height of 4.5 feet (140 cm) above the ground in the United States, or 1.3 meters in Canada and many European countries. DBH is measured up high to avoid measuring a tree's butt swell. Butt swell is where the base of the tree is unconventionally thicker than the rest of the tree. Height and diameter are used to determine the volume of a given tree; measuring above the butt swell is required to provide the most accurate measurement.
Diameter is easily measured because the tape is calibrated in units of 3.14 (pi) inches or centimeters. The tape is actually measuring the tree's circumference but since circumference and diameter are related by pi, the diameter can easily be obtained. The only problem with the diameter tape is that the measurements assume the tree's trunk cross-sections are perfectly circular. Thus the diameter tape just gives an approximation; regardless it is the most commonly used dendrometer for DBH.
Precision diameter tapes are used for measuring the true diameters of both round and out-of-round forms. Used in the metalworking industry, these tapes are precision tools made of 1095 clock spring steel.
The precision-diameter gages consist of a narrow metal ribbon bearing special graduations. These graduations are designed to convert circumferential distance into diameter measurement so the observer can read the diameter directly from the scale.
The tapes are checked over master gages at 0.001 in (0.025 mm) accuracy for standard tapes up to 144 inches (3,658 mm).
To measure the diameter of a tree, the Diameter tape (diameter side facing user) is wrapped around the tree, in the plane perpendicular to the axis of the trunk at 4.5 feet (1.4 m) above ground. Where the number "0" aligns with the rest of the tape, this is the diameter of a relatively round and smooth tree trunk as shown.
See Tree girth measurement for some cautions about placement, and errors that may be introduced, by reporting this diameter for a trunk, or any material, that is not nearly circular.
- Moran,L.A, and R.A Williams.2002. Comparison of three dendrometers in measuring diameter at breast height. Northern Journal of Applied Forestry 19:28-33.