(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.
Ina
that vol
reduce «
type an
The
as fine €
filter co
timber 1
the groi
and like
motion
bered re
scale, tir
Sca
four shc
of those
is obtair
The
available
chromat
The
advanta
Ma
processir
It has b
can now
shows F
referred.
results f
variable
FOR
The
meditati
particul.
We
adverse
interest.
Un
some of
they are
weather
Th
perfect
monum: