306 IMPROVING THE ACCURACY OF TIMBER ESTIMATES 



2. The trees may not be counted accurately. This error is identical with the 

 first, but usually shows up as a deficiency of small timber near the minimum diameter 

 tallied. 



3. The diameter of the trees may be over- or under-estimated either as a whole, 

 or in certain classes. There is a strong tendency to bunch diameters towards a tree 

 whose size seems to be the standard in the cruiser's mind. This results in over- 

 estimate of small trees and under-estimate of trees of larger diameters. 



4. The heights may be over- or under-estimated. When this happens it shows 

 up consistently for the whole tract, the standard of height apparently being tem- 

 porarily distorted in the mind of the cruiser. 



A fifth source of error, the volume table and the failure to coordinate upper 

 diameters and merchantable lengths with the standard used in this table, serves 

 to exaggerate the per cent of error in the judgment of heights, but is always indi- 

 cated when the average heights are too high or too low to agree with the measure- 

 ments. When the volume of the average tree is high or low, it usually means an 

 over- or under-estimate of diameters or heights. The exact character of the error 

 in diameter and height is ascertained by a simple check as follows : the cruiser com- 

 pares the number of trees in each diameter class with that of the standard record and 

 sets down his difference plus or minus. If he is over-estimating, but has the right 

 number of trees, the minus sign will appear opposite the smaller diameters and the 

 larger diameters will show excess numbers. If under-estimating, the plus signs 

 will appear opposite the small diameters. The same rule applies to heights. An 

 over-estimate causes minus signs to appear opposite the lower height classes and 

 corresponding plus numbers in those of greater log lengths. The size of these dis- 

 crepancies shows the degree to which the error has been carried. 



It is the tendency in cruising as in scaling logs, in an effort to correct a known 

 error, to incur immediately a still greater error in the opposite direction; but when it 

 is possible to check against a measurement which the cruiser admits is infallible and 

 in which he has confidence, this tendency to fluctuation is soon overcome and rapid 

 improvement is noted, not only in the total per cent of accuracy which is sometimes 

 merely the result of large compensating plus or minus errors, but in each of the four 

 elements of accuracy, thus insuring a consistent degree of accuracy from day to day. 



The cruiser is expected to master but one detail at a time, and the schedule 

 is as follows: 



1. During the calipering of the standard plots, the eye is trained in estimating 

 diameters which are then promptly checked by the measurements. The same is 

 true of heights. 



2. The second period is devoted to a total or 100 per cent tree by tree estimate 

 with a tally of each diameter and merchantable length. The total area of the 

 plot is covered by eight strips, 5 rods wide, the crvuser working not in the center, but 

 on one side of this strip with compassman marking the opposite border. Width of 

 strip and success in getting 100 per cent of the area is dependent absolutely upon 

 use of eye, checked by pacing and judging distance, and the men are not permitted 

 to mark the boundaries of these strips to prevent overlapping. Twenty acres per 

 day are covered by this method. 



3. The third step is to increase the area covered per day to 30 acres by doubling 

 the width of the strip to 10 rods, the cruiser taking the middle of the strip and judging 

 5-rod distance on each side. In all of this work, the cruiser tallies his own dimen- 

 sions of the trees. In these preliminary 100 per cent estimates, constant repeated 

 checks are made of the diameters and heights to contiiuic the imj^rovcment of the 

 eye. 



4. The 100 per cent estimate is continued, but the tally of every diameter is 



