28 NJAF 19(1) 2002 Diameter at breast height (dbh), measured at 4.5 ft up the bole of the tree, is the most commonly measured tree attribute, whether for inventory, management, or research purposes. The word “diameter” implies that trees have circular cross-sections. However, various studies have dem- onstrated that tree cross-sectional areas more often repre- sent closed convex areas rather than circles (Williamson 1975, Monserud 1979, Kellogg and Barber 1981, Chacko 1961, Biging and Wensel 1988). The three most commonly used tools, or dendrometers, for dbh measurement—the diameter tape (d-tape), calipers, and Biltmore stick—use different geometric relationships to estimate tree diameter resulting in different dbh values for these irregularly shaped trees. The d-tape is a steel or cloth tape that is wrapped around the tree’s circumference, and is graduated into diameter equivalents using the relationship circumference = π × diam- eter. Thus, the diameter inch equivalents are marked on the tape at intervals of 3.1416 in. to a precision of 0.1 in. Deviations from “true” circumference of the tree result from rough bark and surface irregularities. The magnitude de- pends on bark characteristics of the tree and species and the NOTE: R.A. Williams can be contacted at (614) 688-4061; Fax: (614) 292- 7432; E-mail: williams.1577@osu.edu. Copyright © 2002 by the Society of American Foresters. Comparison of Three Dendrometers in Measuring Diameter at Breast Height Leigh Ann Moran, Columbus Wood Products, 1165 Kinnear Road, Columbus, OH 43212-1162 and Roger A. Williams, School of Natural Resources, 210 Kottman Hall, The Ohio State University, 2021 Coffey Road, Columbus, OH 43210-1085. ABSTRACT: Three dendrometers—d-tape, calipers, and Biltmore stick—were used to measure diameter at breast height (dbh), and discrepancies that occurred among these instruments were compared. Three methods of dbh estimation with calipers—the quadratic, arithmetic, and geometric mean of the major and minor axis diameter—were compared. Trees were grouped into four broad dbh classes of 1–5, 6–10, 11–15, and 16–20 in. and three species—northern red oak, sugar maple, and white ash—to determine the effect of tree size and species on discrepancies. The d-tape consistently recorded a larger dbh than the three caliper methods, but was not statistically different nor practically important. The differences in recorded dbh between the d-tape and calipers increased with tree size and were similar among northern red oak and sugar maple trees, but dbh differences in white ash trees were significantly less than in the other two species. The Biltmore stick’s accuracy in classifying trees into the same dbh class as determined by the d-tape decreased as tree size increased. When examined by species, the Biltmore stick was less accurate in this regard with northern red oak and most accurate with sugar maple. Because the geometric principle of the d-tape assumes a tree to have a circular shape, its diameter estimation and subsequent basal area will usually be greater than the true diameter and area. The use of calipers reduces this bias, but the differences are not statistically significant. North. J. Appl. For. 19(1):28–33. Key Words: Calipers, d-tape, Biltmore stick, dbh measurement. extent of surface irregularities. When irregular trees are measured with d-tapes, convex deficits occur where the tape passes over areas of the tree surface that have depressions. The resulting diameter measurement will produce a basal area that is larger than the true basal area because the d-tape assumes the circumference of the tree measured is a circle. The basal area will be greater than the true area because a circle encloses more area with a given perimeter than does any other plane figure (Brickell 1970). For this reason, if bias in the circumference measurement occurs, it will always be positive. Calipers consist of a graduated scale on wood or metal at 1 in. intervals, usually to a precision of 0.1 in. One arm is fixed at the origin of the scale, with the other arm movable along the scale. To read correctly, the movable arm must form a 90˚ angle with the scale when the two arms are pressed together. The diameter can be read directly from the scale. Often two diameter readings are recorded for each tree with the use of calipers: the major axis (estimated widest diameter of the tree) and minor axis (diameter measurement at a right angle to the major axis). It is common practice to simply take the arithmetic mean of the two diameters. However, very few trees have circular cross-sectional areas, particularly as trees become large. Some trees may approach an elliptical shape in cross-section, in which case the geometric mean should be used. If the cross-sectional shape of a tree is either circular or elliptical, the geometric mean will yield results with less bias. Field Note