260 ESTIMATION OF STANDING TIMBER 



the selection of what he believes to be a tree of average diameter and 

 height whose volume he then ascertains. There are two modifications 

 of this system, dependent upon whether the unit used is the log or 

 the tree. When the log unit is used, the cruiser estimates the number 

 of logs in the average tree and the contents of the average log or log 

 run (§ 120). 



In the above three methods of averaging, nothing has been said 

 about the question of area covered. The averages apply to that portion 

 of the area on which the timber is either counted or in addition is tallied 

 for dimensions. This may be 100 per cent or the total area. Although 

 it may not be possible to measure, by diameter and total height, each 

 tree on the entire area, yet by the employment of one of these three 

 methods of averaging the contents, all of the trees may actually be 

 accounted for. 



The remaining three of the six methods of employing averages 

 apply to tracts whose area is too large to permit of 100 per cent esti- 

 mates, even by the simplest plan of counting and obtaining the average 

 tree. The principle here is to estimate the stand on a portion of the 

 area in an effort to derive the volume of the stand upon the remainder. 

 The systems used are as follows: 



4. The stand per acre is guessed at or estimated by eye. This stand 

 multiplied by the area in acres presumably gives the total stand on the 

 tract. This is merely a modification of the method of total ocular 

 estimate, in which the problem of arriving at the average is approached 

 in a different manner. It is possible for a skilled estimator to guess 

 closely the stand on a given acre, but the difficulty lies in either finding 

 a specific stand whose volume per acre happens to agree with the aver- 

 age on the entire tract or else to decide from the inspection of given 

 stands how much the actual stand per acre observed on specific plots 

 must be modified in order to obtain the true average for the entire 

 tract estimated. The probabilities of error in estimates made on this 

 basis increase with the size and diversity of the stand to be estimated. 



5. The dimensions and volume of the trees on a given per cent of 

 the total area are obtained by one of the first three methods and the 

 stand thus found is assumed to represent the average stand per acre 

 for the entire tract. This requires, first, the accurate determination 

 of the area of the tract and of the area covered by the estimate, and 

 second, the location of this latter area in such a way that the assumption 

 that it represents the average of the remainder can be accepted as 

 approximately correct. 



6. The same principle is employed as described under 5, but the 

 assumption that the per cent of area so measured will give an accurate 

 mechanical average applicable to the remaining tract is not accepted. 

 Instead, the remainder of the area is inspected by the method of ocular 



