This function provide correction for basic volume estimation using cylinder formulation v = DBH^2/4 π H. Factor form is given by taking ratio between real volume and apparent volume.
1 
volume 
volume of a log, can be the output of

dbh 
diameter at breast height (1.3 meters from floor) 
height 
commercial height, length of stem or whatever length of log you used in your estimations of cylinder volume 
form factor ranging form 01 (numeric value)
http://wiki.awf.forst.unigoettingen.de/wiki/index.php/Stem_shape
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