MEASUREMENT OF UPPER DIAMETERS. ' DENDROMETERS 247 



200. The Measurement of Upper Diameters. Dendrometers. 

 Upper diameters of standing trees must be measured, first, in estimating 

 timber to a merchantable top diameter; second, to determine the form 

 quotient of the tree, where form classes are to be distinguished. 



In timber estimating, ocular methods are used entirely, and the 

 probable upper diameters approximated by knowledge of rates of taper 

 checked by the measurement of diameters on the boles of down trees. 

 But for the measurements required to determine form quotients, it is not 

 safe to depend altogether on chance windfalls, nor can cutting sample 

 trees be resorted to on account of the time and expense involved. The 

 eye is not sufficiently accurate to gage diameters at upper points, hence 

 these measurements for form quotient must be taken on standing trees 

 by instrumental means. 



An instrument intended to measure the upper diameters of stand- 

 ing trees is termed a dendrometer. 



The principle of the dendrometer is that of similar triangles; but in this case 

 two sets of triangles are used, first, those required in determining the height to the 

 point to be measured, 

 and second, those 

 used to measure the 

 diameter at this point 

 by comparison with 

 the side of a smaller 

 triangle on the 

 dendrometer. These 

 principles are illus- 

 trated in Fig. 54. 



In determining the 

 form quotient for 

 standing timber, 

 either according to 

 Jonson's or Schiffel's 

 methods, the diam- 

 eter at the middle 

 point, either above 

 D.B.H. or above the 

 stump respectively, 

 is sought. As point- 

 ed out, the absolute 

 form quotient cannot 



be determined with scientific accuracy from measurements taken outside the bark 

 or on standing timber, but approximate results can be obtained. 



The triangles whose bases are respectively B, 6i and 6 2 are similar, and the 

 relation between B and either 6i or b 2 determines the diameter at B. But the 

 points bi and 6 2 are not the same, and this difference distinguishes two different 

 principles used in constructing dendrometers. 



When the distance Ac to the horizontal scale on which will be read the upper 

 diameter B, is fixed, so that on sighting at point B this distance coincides with b 2 , 



Fig. 54. — Principles underlying construction of dendrom- 

 eters, as illustrated by the Biltmore pachymeter. 



