20 INSTRUMENTS USED IN MENSURATION. 



The inaccuracy of the better hypsometers does not exceed 

 2 per cent, of the height of the tree. 



b. Trigonometrical Height Measuring. 



This is based upon the measurement of the angles of eleva- 

 tion and depression indicated by rays running from the eye 



4, 



Fig. 12. 



of the observer to the top and foot of the tree. In A B C D 

 (Fig. 12) :- 



E G = D C x tan. b 

 and in A D C A, 



hence, 



A G = D G x tan. a 

 C = H = DC (tan. a + tan. b). 



If the horizontal line of vision passes below the foot of the 

 tree, the above formula becomes : 



H = D (tan. b - tan. a). 

 If it passes above the tree, 



H = D G (tan. a - tan. b). 



Ill each of these cases, the measuring of the horizontal line 

 D C can be avoided by placing a staff of known length along- 

 side the tree. In that case (Fig. 13) : 



