264 



ESTIMATION OF STANDING TIMBER 



Standard distances between strips or plots are 80 rods, or once 

 across a forty for very extensive work of low accuracy; 40 rods, or 

 twice across a forty for work of average accuracy; 20 rods, or four 

 times across a forty for work approaching a 50 per cent estimate; 

 10 rods, or eight times across a forty, which with a 10-rod strip 

 permits 100 per cent of the timber is to be measured. 



The first problem then, in estimating a tract, is to decide upon the 

 proper per cent of the area which must be covered to secure the desired 

 standard of accuracy, and this per cent will be a direct function of the 

 size of the smallest unit of area upon which a separate estimate is 

 required (Fig. 57). 



Sq.Mile 



1 Square Mile 



TTTTT 



bii 



25 Square^liles 



■*++ 



'4 Mile 



U 



IMile 



-4 



Fig. 57. — Influence of size of tract upon probable error in obtaining average volume 

 per acre, by running strips 40 rods apart in each instance. Dotted lines 

 indicate location of strips. 



Narrow strips spaced at one of these standard intervals are commonly 

 used for large tracts. Upon small tracts, the necessity for increasing 

 the per cent of area covered, as a substitute for increasing the number 

 of strips run, takes the form of widening the strip. This is usually 

 accompanied by a modification of the method of tallying the trees 

 and the substitution of a count for the measurement of every diameter. 

 For small areas as low as 40 acres, this frequently takes the form of a 

 100 per cent estimate, the strips being so arranged that they cover the 

 entire area, and where the value of the timber and its size is such that 

 accuracy is desired for each forty, 100 per cent of the entire tract is 

 covered, no matter what its total size. 



The relations between the distance apart of strips or plots, width 

 or size of these strips or plots, and resultant per cent of area covered, 

 to the size of the unit of area to be estimated, is the most practical 



