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- 1827 - 
The figures of Table 5 indicate that the difference in office man-day 
units between the unrestricted and the two-strata sampling method is 
very small. 
The gain in the number of field days in the stratified sampling method is 
outweighed by the extra office man-days. Of course the difference would 
be greater for a higher number of plots, which would occur for a standard 
error of the mean smaller than 5%. 
Two-phase sampling seems to be most eificient. A drawback of this method, 
however, is the correct location in the field of the randomly pricked 
photo plots, which requires a photographic scale between 1:5,000 and 
1:10,000 for stands of > 41 years. 
For two-stage sampling a scale smaller than 1:10,000 can be used. When, 
in addition, selection of secondary plots is done systematically, this 
method, in spite of the higher office man-day units, may become more 
interesting than the two-phase sampling. 
In the example, extra photographs at a scale of 1:10,000 (the smallest 
allowable scale) are already used for the two-phase sampling in addition 
to the 1:30,000 scale photographs. Flying 1:30,000 scale photographs and 
constructing 1:10,000 scale orthophoto mosaics is cheaper than the 
construction of 1:10,000 scale orthophoto mosaics from 1:10,000 scale 
photographs. The difference in cost for the described area including 
expenses for flying, photo material, aerial triangulation, ground control 
and detail transfer, amounts to approximately 12,200 US$. 
Therefore, considering only the additional flying plus the photographic 
material costs for 1:10,000 scale photography, the extra expenditure 
approximates $1476.- which is equivalent to about 18.5 man-days (at 
$10/man hour). 
For the two-phase sampling, instead of 16.8 (Table 5), now 35.3 office 
man-day units are needed, which is higher than the 25.5 (Table 5) as 
calculated for the two-stage PPS sampling. 
The advantage, expressed in man-days, of two-stage PPS sampling over 
unrestricted sampling equals 31 per cent. The advantage Of two-phase 
sampling over unrestricted sampling reduced from 37 to 5.5 per cent after 
considering the additional expenses for the 1:10,000 scale photographs. 
When, in addition, the area is als0 larger in size than 10,000 ha, it will 
be more unlikely that the 1:30,000 and the 1:10,000 scales are flown in 
one day. In this case, costs must be added for addittonal flying from the 
airbase to the target area, double trip, at a rate of $400/hr. 
The resulting additional cost or number of man-days then depends on the 
distance from airbase to target area. 
References 
D.A. Stellingwerf, Results of two-stage unequal probability (PPS) 
1978 sampling for timber volume using an orthophoto 
mosaic. (Comm.VII-ISP-Freiburg 1978; IUFRO, SG.6.05 
Freiburg 1978) 
F. Loetscn, Z. Zührer, Forest inventory, vol. II, 469 pp. 
K.E. Haller, 
1973 
Trân-Châu-Lam and Efficiency of several sampling methods 
A.A. Khan, (M.Sc. Studies ITC) 
1977 
M.H. Hansen, W.N. Hurwitz, Sampling; survey methods and theory, vol. 1. 
W.G. Madow, Methods and applications, 639 p. 
1953