GIRTH AS A SUBSTITUTE FOR DIAMETER 25 



One advantage of girth measurements over diameter is that two 

 measurements taken at the same point give consistent results, while 

 in determining the average diameter of large and irregular or eccentric 

 logs, considerable differences may occur in two separate measurements. 

 Owing to the difficulty of measuring the girth of a log at its middle 

 point, the mean of the two ends may be taken. This incurs an error 

 identical with that by the mean diameter method (§29). This error 

 is offset by the tendency of girth measurement to over-run. 



The volume of the cylinder whose basal area is obtained from girth 

 may be found by the method of the Fifth Girth in which 



•=(f) ! 



i^) 2 2h- 



G is here expressed in feet. If measured in inches, divide the result 

 by 144. Another method, known as the Quarter Girth, is expressed as 



-(f)- 



113. 



In this formula G is expressed in inches. 1 



1 The Fifth Girth method will give a result which is only approximately correct. 



G=tD, 



ttD 2 (vD\ 2 



—-ft should equal I — 1 2ft, 



therefore, 



and 



7T (IT \ 2 



— should equal I - X2, 

 4 \5/ 



.7854 should equal ,6283 2 X2, 



.7854 should equal .7895, 



an error of less than 1 per cent. 



The Quarter Girth formula is of no particular value as it is merely a means of 

 correcting a commercial standard (§35 Hoppus or Quarter Girth Log Rule) to 

 obtain the full volume of the cylinder. 



