234 THE MEASUREMENT OF STANDING TREES 



D.B.H. class, instead of the average size, as 14 inches, enabling the cruiser to 

 classify accurately all trees on the border line between two diameter classes. 



In measuring trees of eccentric or irregular cross section, the errors incident to 

 caliper measurement are exaggerated by the use of the Biltmore stick, but as before, 

 these errors tend to compensate and can be neglected. 



BrUce has suggested that the volume tables standardized at D.B.H. should be 

 converted to values for diameter at the height of the eye, or D.E.H., standardized 

 at 5 feet 3 inches. To do this, taper measurements are taken to establish the 

 D.E.H. of trees of given D.B.H. By interpolation, the volumes corresponding to 

 given even D.E.H. inches can easily be obtained. 



In the ordinary use of the Biltmore stick, it is necessary to bevel the edge 

 opposite the figures so that the measurement may be taken in contact with the bole. 

 Otherwise the thickness of the stick reduces the distance from the eye and incurs 

 an error whose magnitude is determined by this thickness. By deducting this 

 thickness (£) from the distance (a) in the formula, so that this formula reads, 



Scale = - 



Va(a+D)' 

 the resulting values are correct for the face of the stick. 



192. Ocular Estimation of Tree Dimensions. Where the diameter 

 of every tree on a given area must be recorded, the time consumed in 

 actually measuring the diameters is a considerable item of expense. 

 Except when scientific measurements or permanent plot records are 

 required, estimators plan to educate the eye to read as large a percent- 

 age as possible of the diameters directly without measurement, using 

 the calipers, diameter tape or Biltmore stick merely as a check. This 

 is especially desirable when the cruiser is doing his own tallying. 



While the eye can be trained with considerable rapidity to a sufficient 

 degree of accuracy for estimating, it is constantly liable to error and 

 must never be relied upon for even a single day without instrumental 

 checks. These should be made on starting work and at intervals during 

 the day. The eye may be trained to judge diameters at different 

 distances equally well. Some men develop this faculty more rapidly 

 and to greater degree than others. It is the general tendency in ocular 

 estimation to favor a tree of a given size, diameters of trees of lesser 

 size being over-estimated while larger diameters are under-estimated. 

 The use of 2-inch diameter classes greatly facilitates ocular estimating. 



In training the eye to estimate diameters, the greatest progress is 

 made by repeated guesses followed immediately by the measurement 

 of the tree which is then closely observed to fix the known diameter and 

 correct the faulty observation. Since ocular estimating is not a matter 

 of reasoning but of impression, the decision as to the dimensions of the 

 tree should be made instantly. Otherwise fatigue and consequent 

 inaccuracy ensue. 



