292 IMPEOVING THE ACCURACY OF TIMBER ESTIMATES 



tree in a stand is tallied there is no necessity for average heights. Only 

 when merchantable log lengths are used as the basis for height will the 

 height of every tree measured for diameter be tallied. Where total 

 height is used, far greater accuracy can be obtained by the measure- 

 ment of a few trees with a hypsometer than by attempting to guess 

 by eye the height of each tree. 



In a large tract with varying site qualities, the securing of the average 

 height for each diameter class from a range of heights of 100 per cent 

 would require the selection of heights on the basis of the principle of 

 a weighted average. If exactly the same proportion, as for instance, 

 1 per cent, of the heights for each diameter were obtained from large, 

 medium and short trees as existed in the original stand on the entire 

 tract, the height curve could then be applied to the tract as a whole. 

 Any failure to secure this weighted average would result in a curve 

 giving too high or too low an average for the timber as a whole. 



The difficulty of securing a weighted average is eliminated if the 

 tract can be divided into two or three site qualities, separated as dis- 

 tinct units in the field in estimating. On each of these separate sites 

 the heights conform to a much closer range for the same diameter than 

 for the entire area, and a few selected trees for each class will give a 

 dependable height curve (§ 209) from which the volumes in each 

 diameter class may be accurately computed. 



228. Methods of Estimating which Utilize Types and Site Classes; 

 Corrections for Area. An example of the application of these principles 

 is found in the standard methods of timber cruising adopted by the 

 Forest Service in the Appalachian region. Four types are used, termed 

 cove, lower slope, upper slope and ridge. The variations in the per 

 cent of estimate required are shown in the following table: 



TABLE XLIV 

 Per Cent op Total Area Required in Estimating 



