i68 Forestry Quarterly. 



The accuracy of any given per cent, cruise will depend : 



1. On the uniformity of the distribution of the timber. The 

 more evenly distributed the timber, the more accurate is the 

 estimate, because the sample areas measured are then most likely 

 to be representative of the entire forest. 



2. On the size of the area to which it is applied. While a 

 5 per cent, estimate may give very inaccurate results when ap- 

 plied to a single "forty," it may give quite satisfactory results 

 when applied to an entire section, because the errors in the esti- 

 mate of the individual forties are then compensating, the cruise 

 on one being too high and on another too low. 



Assuming that a given system of estimating is correct and that 

 its application is faultless, the question arises: How small a 

 per cent, of area is it safe to estimate to obtain a satisfactory 

 cruise of a given stand, and for a given per cent, cruise how 

 small an area can it be safely applied to? 



Some light on this question has been thrown by work done in 

 California by the Forest Service during the past year. Here a 

 complete lOO per cent, estimate was made on 2200 acres of 

 forest. By comparing a 5 per cent, and a 10 per cent, estimate 

 on the same area with the 100 per cent, cruise, the errors in- 

 volved can be determined and studied. 



The area of 2,200 acres on which the test was made consists 

 of two parts ; one, an area of 640 acres on the Plumas National 

 Forest, and the second, an area of 1,560 acres within the Yosemite 

 National Park. On the Plumas, the work was done primarily 

 for the purpose of determining the relative accuracy of a 5 per 

 cent, and a 10 per cent, cruise. In the Yosemite, the test was 

 necessary for administrative purposes. The two areas will be 

 discussed separately. 



The Plumas Area. 



The area selected contains 640 acres and is of irregular shape, 

 covering parts of four sections. The stand of timber averaged 

 about 28,000 board feet per acre, and the forest was mainly of 

 the mixed conifer type in which yellow pine and Douglas fir 

 each averaged about one-third of the stand, sugar pine about one- 

 sixth, incense cedar about 5 per cent., the remainder being white 

 fir. Although the topography was not as rough as' is typical on 

 most of the Sierra forests, several exposures were represented. 

 The work consisted of: 



