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Mr. Von Frytag Drabbe, Netherlands: Many factors are involved in the
photographic scale. In the first place the quality of the photographs must be
considered. In the second place the scale required for a given survey depends
upon the type of the trees in the forest. Some species can be recognized by their
leaves, others by the shape of their crown. In the latter case, small scale photo
graphs are sufficient. The third factor is the type of stereoscope. Finally, the
scale required will depend upon the stereoscopic vision of the interpreters for
this varies from person to person.
Mr. Earl Rogers, U. S. A.: In the United States we are having trouble deter
mining the density of forest cover. I believe we need some new equipment and
techniques. Mr. Thunberg showed that you can have an open forest stand,
without being able to see the ground in a stereo pair of aerial photos. I would
like to ask him if he plans to apply his results to the problem of determining the
density of the crown cover, and what recommendations he might have for the
specifications of a photographic camera in order to give us the best results. Also,
does anybody here have experiences in preparing photo specifications that will
insure ground visibility in forested areas?
Mr. A. Thunberg, Sweden: The question of the test camera is a large one and
I have only made a limited investigation as yet. I have only used one forest model,
and I have not taken into consideration neither topography, nor the light passing
through the tree crowns. The best camera would be one with a normal angle
lens. If you wish to use a wide angle lens at least 80 % endlap would be required.
40 % sidelap would generally be sufficient.
Mr. Axelson (Sweden): Negative scale 1: 30 000 is as good usually as 1: 15 000
for forestry purposes in Sweden. We have used the Wild Camera Rc5a.
Mr. Rogers: I am of the opinion that instruments will help foresters do a
better job. The instrument may be the Kelsh plotter or a similar instrument
which may be designed for technical foresters. I have in mind one upon which
we can read area direct on a photo. This is done by measurements on photos so
that areas are estimated directly without map compilation.
I am particularly interested in looking at sample plots of 1/5 or 1/4 acre. In
this area I want to measure the trees, count their number and determine the
kind of trees and other characteristics of the forest. It is difficult to use type of
plotting equipment available. We could map all these details but we do not
need a map, all we want is the information within these small plots. If we had
some way of looking at the model and determining areas direct, this would be
of great help. Also we wish to have built in devices to measure distances, crown
density and parallax. I feel that there is a good market for such an instrument.
Mr. Losee: There is no doubt that these investigations and developments as
suggested by Mr. Rogers are needed. Mapping of aerial photos is way ahead
of us in that respect and we have not made full use of our opportunities in
utilizing their developments.
Mr. Coleman: The utilization of such an instrument is not limited to only
forest photo interpretation. I have noted with interest during the last year that
the procedures the people in forest interpretation have developed for analyses
using sampling of forest stands are similar to those used in urban area analyses.
I believe that the instrument suggested by Mr. Rogers would not be confined
to forest photo interpretation only.
L’échelle de la photographie pour l’interprétation forestière est discutée. En Suède on emploie
des échelles entre 1/15 000 et 1/30 000, et des études expérimentales ont été effectuées sur celles
de 1/8 000 et 1/15 000. Au Canada les échelles plus grandes comme 1/8 000 sont préférées.
Concernant la détermination de la densité des forêts, l’expérience suédoise indique qu’un objectif
à angle normal en fournira la meilleure couverture.
La grande valeur d’instruments éventuellement construits spécialement pour l’interprétation
forestière est discutée.
