A Review of Past Research on Dendrometers
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A Review of Past Research on Dendrometers Neil A. Clark, Randolph H. Wynne, and Daniel L. Schmoldt ABSTRACT. The purpose of a dendrometer is to measure tree diameter. Contact and noncontact dendrometers accomplish this task by collecting different metrics, including girth or distance between tangent points on a tree stem. Many dendrometers have been developed in the last quarter century and many have been retired. This article summarizes instrument developments and application results, contains an interpretation of the results, and provides guidance for dendrometer selection. FOR. SCI. 46(4):570–576. Additional Key Words: Instrumentation, diameter measurement, forest inventory, mensuration. IAMETER MEASUREMENT IS AN IMPORTANT PART of most facture, design, and operation have left them immutable since forest analyses. Many types of diameter measuring their inception with the only significant technical advances D instruments (dendrometers) exist, possessing widely coming in the form of digital recording devices. Arguments differing properties (e.g., accuracy, precision, cost, opera- about the relative merits between tapes and calipers have tional simplicity, etc.) Yet considerable time has elapsed gone on for years, and the following conclusions have been since a state-of-the-art assessment of dendrometers has been affirmed. These instruments acquire two different metrics. published. Grosenbaugh (1963) and Brickell (1976) present Calipers measure the distance between parallel tangents of a very thorough coverage of dendrometer history, develop- closed convex region, while diameter tapes measure the ment and evaluation, published to date of their respective perimeter or girth of this region (Figure 1). Diameter tapes reviews. Since these reviews were published, there have been can be said to be more “consistent” (Avery and Burkhart new dendrometer studies but no summary works. Given this 1994, p. 97) than calipers as the measurement represents an time lapse, the acceleration of technological advances, and average of all diameters over all directions, thus eliminating new devices being studied, a review is needed. This report variability caused by direction. However, it is now recog- provides a summary of more recent studies, introduces cau- nized that departures from the assumptions of convexity and tions about comparing independent studies, and indicates circularity impede the simple attainment of the elusive “di- guidelines for dendrometer selection. ameter.” If used properly, both tools provide comparable results with the majority of bias caused by mathematical Contact Dendrometers models that do not accurately represent stem cross sections (Brickell 1970, Biging and Wensel 1988). More on cross- Dendrometers can initially be divided into two categories: sectional geometry and related concepts can be found in those that contact the stem physically and those that obtain Matérn (1990). measurements remotely. Conventional calipers and diameter The electronic tree measuring fork (ETMF) (Binot et al. tapes [included are dendrometer bands (Keeland 1993) and 1995) is the only other true contact instrument. The ETMF rubbery rulers (Costella 1995)] are the primary “contact” has two arms, 60˚ apart, that contact the tree. Diameter is dendrometers used by foresters. The simplicity of their manu- computed by measuring the speed of ultrasonic waves from Neil A. Clark is Research Forester, Integrated Life Cycle of Wood: Tree Quality, Processing, and Recycling, USDA Forest Service Southern Research Station–0503, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061—Phone: (540)231-4674; Fax: (540)231-8868; E-mail: [email protected]. Randolph H. Wynne is Assistant Professor, Department of Forestry, College of Natural Resources, Virginia Polytechnic Institute and State University, 321 Cheatham Hall–0324, Blacksburg, VA 24061—Phone (540)231-7811; E-mail: [email protected]. Daniel L. Schmoldt is Research Forest Products Technologist, Integrated Life Cycle of Wood: Tree Quality, Processing, and Recycling, USDA Forest Service Southern Research Station–0503, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061—Phone: (608)262-3313; E-mail: [email protected]. Acknowledgments: The authors thank the reviewers for their time and valuable suggestions for improving the manuscript. Manuscript received May 3, 1999. Accepted April 10, 2000. This article was written by U.S. Government employees and is therefore in the public domain. 570 Forest Science 46(4) 2000 Figure 1. Comparison of caliper versus tape derived cross- a b c sectional area estimates (assuming a circular model). Deviation from an assumed circle will always result in a positive bias Figure 2. Comparison of principles of three types of optical ( Dˆ - D > 0 ) when using taped measurements and introduce an dendrometers. (a) For the optical caliper, the baseline distance unpredictable directional variation in caliper measurements. (b) is adjusted and substituted as a direct measurement of diameter (distance is not necessary). (b) For the rangefinder, b is a transmitter to a receiver. In the study by Binot et al. (1995), typically fixed and true and false convergence angles are used to calculate distance and ultimately, diameter. (c) Using an optical diameter results were comparable to calipers and diameter fork, two distances ( d1 and d2) and a baseline distance (b) are tapes with a 35 to 40% time savings. Concerns were noted required for diameter. regarding bias caused by signal interference induced by bark characteristics of some species. garding vision restrictions). Direct readings of parallel tan- A plethora of other devices [e.g., the Biltmore stick gents require only aspect to be controlled when making (Jackson 1911), sector fork (Bitterlich 1998), Samoan stick measurement comparisons with conventional calipers. Some (Dixon 1973), etc.] are hybrid instruments that must both of the early optical calipers were noncoincident (Clark 1913) contact the stem and be interpreted visually. These are based and experienced difficulties maintaining parallelism, but on the optical fork principle, which will be discussed later, instruments using pentaprisms (Wheeler 1962, Eller and with the difference being that the distance from the stem is Keister 1979) or parallel mirrors (McClure 1969) have suc- measured by physical contact. Studies by Matérn (1990) ceeded in producing excellent results. Diameters are limited found these instruments all to have a positive bias, increasing to the instrument’s length (usually 91 cm), though longer, proportional to diameter, compared to conventional caliper less portable, versions can easily be constructed (Grosenbaugh measurements. Although these tools are very handy and 1963). probably the most common instruments among practitioners, A number of empirical studies have been performed with their reliability and subjectivity generally preclude their use pentaprisms since their resurgence in the early 1960s. Wheeler in scientific or large-scale inventory work. (1962) measured ten trees at two heights (1.4 and 5.3 m) using a Wheeler’s pentaprism caliper. Measurements were within ±13 Noncontact/Optical Dendrometers mm of wooden caliper measurements using a 95% chi-square test. Robbins and Young (1968) compared three dendrometers Unlike contact dendrometers, optical dendrometers are to conventional calipers: the Wheeler pentaprism, an early devices that do not require the stem to be approached. Many version of the McClure pentaprism (that only permitted direct styles of optical dendrometers have been designed based on readings to the nearest 13 mm), and a diameter tape. Twenty the fork, caliper, and rangefinder principles (Grosenbaugh stems were measured, containing marked diameters at 6 heights 1963). To measure a diameter optically, two lines of sight (from 1.5 to 10 m). The ranges of errors relative to conventional must exist between the observation location and two tangents calipers were –18, 33, –18, 20, and –20, 25 mm for diameter tape, on the stem lying in the plane representing the desired McClure, and Wheeler pentaprism, respectively. All instru- diameter. Perspective geometry utilizing various angle and ments studied had a slight positive bias between 3 and 5 mm. In distance measurements is then used to calculate the diameter a second study on ten trees at eight heights (1.5 to 10 m), Robbins of the stem in this plane. The following sections discuss each and Young (1973) found the McClure pentaprism produced type in further detail, and Table 1 shows summary informa- greater average differences than the Wheeler pentaprism and the tion for some empirical studies using these optical Barr & Stroud1 dendrometer. The Wheeler pentaprism demon- dendrometers. strated a time savings over both of the other instruments and has Optical Calipers Optical calipers use two parallel lines of sight to view 1 The use of trade or firm names in this publication is for reader information points on a stem that represent the diameter (Figure 2a) and does not imply endorsement by the U.S. Department of Agriculture of making measurement precision distance invariant (disre- any product or service. Forest Science 46(4) 2000 571 Table 1. Details of selected empirical dendrometer studies. Diameter Height # # Distance range range Location Investigator/s Instrument Accuracy results stems obs Genus (m) (cm) (m) method Optical calipers Wheeler (1962) Wheeler 13 mm1 10 40 Pinus NA 16–41 1.4–5.3 NA pentaprism Robbins & Young Wheeler & (–17.8, 33.0) mm2