774 JOURNAL OF FORESTRY 



65 



The height of the tree is obtained by merely adding the instrument 

 readings and multiplying by the horizontal distance (120 feet) used 

 and then pointing off two decimal places. In other words, the rela- 

 tionship between the reading on the hypsometer scale and the hori- 

 zontal distance used is constant — that is. the product is always the 

 same. Hence, when the product is divided by 100 the true height of 

 the object measured is obtained. 



In view of this relationship it is not necessary to make allowance 

 for slope at the time field data for tree heights are taken. Following., 

 is an example : 



Distance of observer Horizontal Instrument w.,vv,t of tr^^ f.^t 



from tree (measured distance, feet readings, feet height of tree, feet 



along slope), leet 



100 98 90 73-5 



—15 



75 



The height of the tree is computed as follows : First determine 

 the horizontal distance, which is derived from the geometrical relation- 

 ship of the hypotenuse (distance measured along slope) to the side of 

 the right triangle (distance from base of tree to zero point recorded on 

 hypsometer scale, plus 5 feet).^ In the example cited the side of the 

 right triangle is 15 f eet + 5 feet ^20 feet and the base is therefore 

 98 feet. (See figure 2.) 



In making computations to determine the relationship between side 

 and base of the right triangles, a table can be previously prepared for 

 convenience. Having secured the correct horizontal distance, the true 

 height of the tree is determined as described under the first example 

 cited. 



Since readings with the Forest Service Standard Hypsometer can be 

 taken at any distance (within the limits indicated by the instrument's 

 scale) its practical value is also enhanced. This applies especially in 

 the case of dense stands, where it is often difficult to locate a point 

 exactly 100 feet (or multiples thereof) distant from the tree and at 

 the same time be able to see the top. Furthermore, since several read- 



^ Five feet are added to the minus quantity because distances are measured 

 between position of observer's eye and same point (5 feet) above base of tree. 

 In the case of plus distances (indicating that readings were taken up slope from 

 Ijase of tree) the 5 feet factor is subtracted. 



