(530) 
Another means of solving problems of this nature is achieved through the 
use of statistical methods. The analysis of variance and Chi-square tests have 
been used in some cases and it is this approach I would like to see pursued 
further. One of the experiments performed under such a statistical design had 
the objective of finding whether there was a significant difference in measuring 
tree heights on glossy prints versus transparencies. The solution was unsatis- 
factory, because despite a long period of training the men performing the 
work were still not sufficiently skilled in measuring tree heights with a paral- 
lax bar and the error variance was so great that no significant results could be 
obtained. It was found as the result of the experiment that the average inter- 
preter must make about 1000 trial measurements before attempting work of 
this nature. It was further found in a parallel experiment that transparencies 
were superior to semi-matte positives. 
Another problem performed by a different agency was designed to find 
what scale was best using forest type recognition as the factor of measurement. 
The results of this experiment are not conclusive. One difficulty was, to the 
best of my understanding, due to lack of training of the interpreters, not from 
a general standpoint but rather that they were not adequately trained to inter- 
pret photographs at scales other than within the range of 1/15,000 to 1/25,000. 
A third problem of this nature performed by another agency likewise was 
designed to obtain information on scale but with tree volume as the basis of 
measurement. The difficulties encountered here were the same as in the pre- 
vious experiment but there was also some misunderstanding regarding the 
flight plan and the areas were not flown at the same time of year. Further 
experimentation by this same agency with continuous strip photography has 
indicated that this means is not at present satisfactory for our purposes. 
Suggested Approach. 
The suggested approach is statistical, for the most part by means of the 
analysis of variance and in the preliminary stages by means of a factorial type 
of experiment. The factorial type makes it possible to try out several series of 
factors at the same time and is particularly well suited to a pilot type of 
experiment. 
We may list the more important considerations that have been mentioned 
or implied up to this point. 
1. The most desirable situation is that of being able to cruise timber directly 
from aerial photographs without resorting to expensive ground work. 
2. As this has not as yet been possible, some ground work is a necessity in 
order to obtain data for the aerial volume tables. 
3. Volume tables should be made for each forest type and other subdivision 
recognized on the aerial photos. 
4. The finer the subdivisions recognized, the more accurate will be the final 
volume estimate (stratified sampling). 
5. As ground work is always a necessity for volume table construction, 
certain additional data can be obtained at the same time. 
6. "This additional information includes data on defect, the percent of each 
different species in mixture by volume or number, timber quality, and 
Whether or not there is an understory of trees. 
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