THE TECHNIQUE OF MEASURING HEIGHTS 245 



This same method was used to determine the value of -de for a 25-, 30-, 35-, 

 40-foot tree, etc., up to 150 feet, and the proper graduations made on the scale. 

 The scale is somewhat more easily read when a notch is made at each graduation. 



The instrument is light and compact, and with practice can be used very rapidly, 

 provided one has an assistant to manage the 10-foot pole. It requires no measure- 

 ment of distance from the tree, and the height is obtained by one observation. 

 It is more rapid than either the Faustmann or Weise instrument. 



Its disadvantages are that it requires a very steady and practiced hand to secure 

 accuracy, that it cannot be used accurately for tall trees, and that it is not adapted 

 for steady work because it is extremely tiresome to hold the arm in the position 

 required. This last objection may be overcome by using a staff to support the 

 hand. 



199. The Technique of Measuring Heights. In rough checks for 

 timber cruising, the distances used in obtaining heights are usually 

 paced. Care must of course be taken to carefully check the paced 

 distance desired to avoid incurring a cumulative error. For the measure- ■ 

 ment of average trees, depended upon to secure the heights of stands, 

 the distance should, if possible, be measured with the tape. This 

 latter method is the only one permissible in measuring the heights 

 of trees on permanent sample plots. 



By the method illustrated by the Klaussner hypsometer, this dis- 

 tance is measured along the ground whether the slope be level, gradual 

 or steep. By the method of right triangles the distance must be meas- 

 ured horizontally to the bole of the tree, and a considerable error would 

 be incurred in measuring it along the surface on very sloping ground. 



Since the entire basis of the similar triangles used assumes that the 

 tree which forms one side of the larger triangle stands in a vertical 

 position, the consequences of measuring a tree which leans either towards 

 or away from the observer are very serious (Fig. 53). 



From the position A, the distance to the base of the tree is AC. 

 But if the observer sights at the tip of the tree B\ which leans towards 

 him, its height, when compared to the distance AC will be read as B'\C, 

 an error of +16 per cent. If the distance is measured instead to the 

 point directly below the tip Bi the height is read as BiCi, with an error 

 of — 2 per cent. Again, if the tree B? leans away from the observer, 

 and its distance is measured as AC, its height will be read as B'-jC with 

 an error of — 16 per cent, but if this distance is measured to the point 

 C2, the height will be read as B2C2 with an error of —2 per cent as 

 before. 1 



If it is necessary to measure leaning trees, this can be done by taking 

 a position at right angles with the line AC in Fig. 53, or at right angles 

 with the vertical plane in which the tree lies. The ocular measure- 



1 Some New Aspects Regarding the Use of the Forest Service Standard Hyp- 

 someter, Hermann Krauch, Journal of Forestry, Vol. XVI, 1918, p. 772. 



