15 



the entire crop, which will rarely be found to be of sufficiently 

 uniform quality and composition throughout. Moreover, the 

 area of a crop is often not exactly known. "When great accur- 

 acy is required, as when the whole of a standing crop is to be 

 put up to sale, it is of course advisable -to measure at least 

 the girth of every tree in the crop. Still it must be un- 

 derstood that, in the most carefully organized and conducted 

 valuation survey, only a limited degree of accuracy is attain- 

 able, for Although the girths or basal sections of the trees 

 may be obtained witli sufficient exactitude, the heights and 

 form-factors of the trees can only be determined approximate- 

 ly. For carefully-framed working plans it is usual to make 

 complete surveys, the procedure by sample areas being adopted 

 only when circumstances render a complete survey difficult 

 and at the same permit of sufficiently correct generalizations 

 from the part to the whole. 



II. THE SIZB AND NATURE OP THE CHOP. Valuation survey by 

 sample plots is obviously admissible only in crops that are so 

 far uniform as to render it practicable to select certain portions 

 presenting the average characteristics of the whole ; but this 

 method of survey is not justifiable if no time is thereby saved. 

 Thus, if a crop is of limited extent, the whole of it can often 

 be surveyed as quickly as a sample plot, which has to be 

 carefully selected and then marked out and measured.. So also 

 in very open crops a complete survey is preferable, as it can 

 be effected rapidly, and the sample plot, to represent the aver- 

 age of such a crop, must be comparatively large. We may 

 lay down the following two rules for general guidance : 



1. In three cases the system of sample plots should be avoid- 

 ed Firstly, in irregular crops of very variable density, or 

 containing trees of very different girths in their different 

 parts; secondly, in small crops not exceeding five acres in 

 extent; and, thirdly, in very open crops, or in crops in 

 which only certain scattered trees, such as coppice stores, 

 large trees in an area under jardinage, have to be accounted 



i ' for. 



2. On the other hand in young crops or in coppice, where 

 often 2,000 and even more stems may stand on an acre* 



