13 



that they need uot be average trees in the strict sense of the won). 

 After the measurements required to obtain the total basal areas 

 of the trees have been taken, it only remains for the surveyor to 

 select his sample trees properly and in suitable numbers for each 

 girth-height class. My suitable numbers is meant a fixed pro- 

 portion of the total number of trees in the respective classes, or 

 equal numbers in case the several classes include more or less the 

 same number of individuals. 



The sample stems are usually felled in order to determine their 

 contents ; but their contents may be" obtained without felling by 

 means of volume-tables or of previously prepared tables of form- 

 factors. 



The methods of valuation hitherto described require that every 

 tree in the forest should be measured. But the contents of the 

 whole crop may also be calculated by means of a sum of simple 

 proportion, from data furnished by sample plots in which alone 

 the trees are measured. 



But measurement of every kind may even be entirely dispensed, 

 giving place either to ocular estimation, or to estimation by com- 

 parison with tesults obtained in similar crops elsewhere. 



The following is a synoptical view of the various methods of 

 effecting a valuation survey of a crop : 



I. VALUATION BY ACTUAL MEASUREMENT. 



A. Of every tree in the entire crop (complete survey). 



B. Of every tree only in sample plots (survey by sam- 



ple plots), 



Whether we undertake a complete survey, or only 

 one by sample plots, we may ascertain the contents 

 of the whole crop 



a By deducing it from the contents of type or 

 sample trees, representing either 



a. The ideal average tree of the crop, or 

 /3. The -average of trees of one and the 



same girth and height, or 

 y. The average of trees of comprehen- 

 sive girth and height-classes (girth 

 and height varying between a maxi- 

 mum and a minimum limit), i.e., girtk 

 and height gradations. 



