231 
Fig. 3 
Standard error and cost paths for photo + field 
and for pure field plots (volume growth determination) 
The combined effect of the two sources of error is demonstrated by the S| growth 
+ volume (n ) line. The S- values are calculated from: 
g g 
S- 2 (n ) = S- + S- . 
g g g v 
The full line indicating the cost path, C (n ) , allows cost calculation for the 
f g 
given field plots (n 
g 
CONCLUSIONS 
n 
The optimum ratio —^ gives either a minimum cost for a given standard error 
n g 
or a minimum standard error for a given cost. In both examples, volume and volume 
growth, the combination of photo and field plots is always cheaper than field plots 
alone. The question whether the saving is really worthwhile, depends on the expen 
diture for the photography and its possible gain when additional applications are 
considered such as map construction etc. for management purposes. 
The difficulty in practice is always to obtain preliminary estimates of the standard 
error of estimate of the regression, of the regression coefficient, of the standard 
deviation of the pure photo variable values and of the cost figures c and c . The 
P g 
answers to these questions must be obtained by either using data from earlier 
experiments or by using a limited number of plots, for example n = 40 . 
g 
REFERENCES 
COCHRAN, W.G. 
1962 
Sampling techniques 
LOETSCH, F. , and HALLER, K. E. , Forest inventory. Vol. 1. Statistics of forest 
1964 
inventory and information from aerial photo 
graphs: 436. 
STELLINGWERF, D. A. , 
1973-3 
Application of aerial volume tables and 
aspects of their construction: pp. 480-498, 
ITC-Journal. 
STELLINGWERF, D. A. , 
1973-5 
Application of aerial photography to gross 
mean annual volume growth determination: 
pp. 820-841, ITC-Journal.