- Diameter tape
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 (137.16 cm) above the ground, where it is most convenient to measure diameter. DBH is measured up high so to avoid measuring a tree's butt swell. Butt swell is where the base of the tree is unconventionally fatter 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 trees 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 a precision tool made of 1095 clock spring steel.precision diameter tapes
The precision diameters 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 .001'"(.03mm) accuracy for standard tapes up to 144" (3600mm). 
To calculate the diameter of a tree, wrap the tape(diameter side facing user) around the tree at 4.5 feet (137.16 cm) above ground. Where the number "0" aligns with the rest of the tape, this is the diameter of the tree.
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