Tree Diameter Measurements 

 At English and IVIetric Standard 

 Heights: A Comparison 



Dwane D. Van hHooser 

 David C. Chojnacl<y 



INTRODUCTION 



On December 23, 1975, President Ford signed into law 

 the Metric Conversion Act. The act calls for voluntary 

 conversion; however, foresters who attempt to comply 

 may be faced with some rather difficult decisions on 

 how to conduct their traditional field inventories and 

 data analyses. 



The Nationwide Forest Survey is a good example. At 

 present, trees are measured to specified merchantability 

 standards. Should these standards be maintained and 

 "soft" conversion to metrics be accomplished through 

 application of various factors or should the standards be 

 changed to conform to even centimeter/meter classes? 

 Similarly, should volume models based on English units 

 be converted or should new metric models be developed? 



Fundamental to many of the questions relating to 

 metric conversion is: Where should diameter at breast 

 height (d.b.h.) be measured? International users of the 

 metric system commonly measure d.b.h. at 1.3 m. Both 

 Australia and Canada, for example, have accepted this 

 point on the stem as their measurement standard. Other 

 countries, such as the United States, have traditionally 

 measured d.b.h. at 4.5 ft. In metric this converts to 1.37 

 m, 7.2 cm higher on the bole than the metric standard. 



In the United States, the Society of American 

 Foresters, working with the American National Metric 

 Council, is proposing that d.b.h. be measured at 1.3 m 

 above ground line (Folliott and others 1982). 



Some ongoing inventories are taking measurements in 

 metric units, including diameter at 1.37 m, and then 

 through conversion computing standard reporting units 

 (Oswald 1979). Most, however, are taking measurements 

 in the usual manner and then showing metric 

 equivalents in the reports. Procedures have been 

 developed to convert English diameter measurements to 

 the metric to produce volume estimates in cubic meters. 

 Bruce (1979), for example, gives the following diameter 

 conversion for West Coast Douglas-fir: 



metric d.b.h. = 1.00513 EngHsh d.b.h. 



Berry (1980) developed another conversion for white 

 spruce: 



metric d.b.h. = -0.049 + 1.01 English d.b.h. 



Bruce also cites a New Zealand study that suggests ap- 

 plying the conversion factor 1.0075 to the coefficient for 

 d.b.h. in volume equations based on English 

 measurement. 



Meyers and Edminster (1974) present a method for 

 converting from" English to metric units volume equa- 

 tions of the form 



Volume = a + bD2H. 



But there is no mention about the effect of diameter 

 measurement taken by English versus metric standards. 



Demaerschalk (1972) also gives procedures for conver- 

 sion of taper and volume equations from English to 

 metric units. Again, there is no comment about the ef- 

 fects from place of diameter measurement. 



In the Rocky Mountains, the English volume equa- 

 tions are in numerous model forms not amenable to sim- 

 ple metric conversion. The most practical approach is to 

 continue taking field measurements in English units, use 

 existing English volume equations, and convert results 

 of the equations to metric units— a soft conversion 

 method. An initial phase in exploring this approach was 

 to determine whether or not there was a significant dif- 

 ference in volume prediction from English equations 

 when using d.b.h. measured at 1.3 m instead of 1.37 m 

 for Rocky Mounteiin species. If the difference was 

 significant, a correction factor for adjusting 1.37 m 

 d.b.h. to 1.30 m d.b.h. for use in soft conversion of 

 English volume to metric volume had to be developed. 



METHODS 



Fieldwork.— Data for this study came from a 

 cooperative timber inventory conducted by the Forest 

 Service and the State of Colorado in Grand County, Col- 

 orado. The inventory design systematically covered the 

 county on a 5 000-m grid. Plots consisted of ten 40-basal 

 area factor sampling points. Diameter of all sample trees 

 was measured at 1.3 and 1.37 m above the ground line. 

 Additional data were collected, but only diameter and 

 height measurements were used for this study. Six 

 species were represented: Douglas-fir [Pseudotsuga 

 menziesii [Mirb.] Franco), Engelmann spruce (Picea 

 engelmannii Parry), lodgepole pine (Pinus contorta 



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