MEASUREMENT BY SECTIONS. 41 



estimated from the diameter at height of chest. The method is 

 a rough one, but much used in Britain, France and Belgium. 

 As far as the author is aware, no uniform method of esti- 

 mating the girth or diameter at half height, from the girth 

 or diameter at, say, height of chest, has been recognised in 

 Britain. Consequently, the actual estimate depends on the 

 individuality of the estimator. No wonder, then, that different 

 estimators obtain different results. The calculation of the 

 volume by means of form factors, which represent the average 

 of numerous measurements of trees lying on the ground, 

 leaves no latitude to the estimator. He measures the dia- 

 meter of the tree at height of chest and the height ; he takes 

 the basal area and form factor out of a little table which he 

 carries in his pocket, and obtains the volume by a simple 

 multiplication. 



Example. — An oak tree, grown in a fairly stocked wood, has 

 a diameter of 12 inches at height of chest and a total height 

 of 70 feet. On reference to the table on page 336, it is found 

 that the basal area, corresponding to a diameter of 12 inches, 

 is s = 0*7854, and on reference to page 39 the form factor for 

 a height of 70 feet will be found to amount to '50 for timber 

 in the round, or '37 (= | of "50) for quarter girth measurement. 

 Hence the volume V = "7854 X 70 X '50 = 27-5 cubic feet 

 in the round, or V = '7854 X 70 X '37 = 20-3 cubic feet 

 quarter-girth measurement. The product of '7854 X 70 can be 

 obtained from the table on page 338, without multiplication. 



On the other hand, according to British custom, the esti- 

 mator has to do three things : (1.) to measure the girth at 

 height of chest, say, = 38 inches ; (2.) the length of service- 

 able timber, say 50 feet; and (3.) to estimate the girth at 

 25 feet from the ground. Supposing he estimates a decrease 

 of 20 per cent., then the girth at 25 feet comes to about 

 30 inches, and the volume, according to the quarter-girth 

 measurement, amounts to — 



144 



