Had the PU areas been determined inconsistantly, the result would have 
shown a higher standard deviation. On the other hand consistantly too 
high or too low areas would have no influence on the results. When the 
values of the standard deviations becomes too high, the method may no 
longer guarantee an efficient application. This may be the case when 
very small scale aerial photographs or satellite images on which the 
interpretation of detail is inconsistant are used. If this is found, 
an improvement in resolution of the existing satellite recording system 
is recommended. On the other hand, the orthophoto mosaic, scale 1:10,000, 
from infra-red photography (1:30,000) was well suited for a consistent PU 
area determination. 
Assuming that a coefficient of the standard deviation (CVs) - column 7 
Table 2 - should not be larger than 13 per cent, according to Hansen, 
Hurwitz, Madow (1953), in order to have a reliable estimate of the standard 
deviation, the combination of SU's of the 225 ha grid still gives acceptable 
results. For 450 ha and 675 ha grid cells the 2 and 5 SU's become doubtful. 
As the CVs are calculated from 35 samples, their values are only an 
estimate and so the values can be considered as a boundary case. 
When a choice must be made from the given combinations, there is a tendency 
to select the one with the lowest standard deviftion, as for a certain 
standard error, a lower standard deviation results in a lower number of 
PU's in the sample. Thé lowest most reliable value is obtained from the 
675 ha grid with 35 SU's/PU (8% = 10.8). For a standard error per cent of 
5: S% = 5, this would result in 
10 8° 
2 = 5 PU's and therefore 5 x 50 = 250 SU's. 
5 
Considering now the 225 ha grid cells with 2 and 10 SU's/PU for 8% = 5, 
respectively 
2 2 
2 
ER = 25 PU's (and 25 x 2 = 50 SU's) and ew = 9 PU's 
5 5 
(and 9 x 10 = 90 SU's), will be involved. It is therefore obvious that 
the 225 ha grid with 2 SU's is selected. 
As sampling is done with replacements, the same SU data of the reselected 
PU’s can again enter into the calculation. For larger areas, reselection of 
PU's should not be expected to occur frequently. For the given 10,000 ha 
forest of Austria, replacements of selected PU's resulted in data which 
are given in Table 3. 
Tabl: 
Redu 
invo 
and 
grid 
225 
225 
x) 
From 
smal 
with 
othe 
An € 
A-pr 
ster 
esti 
volu 
Appa 
and 
devi 
81-1 
41-1 
The 
in t 
age- 
If a 
very 
two- 
lead 
It h 
meth 
conc 
For 
limi 
list 
calc 
Tabl