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Transactions of the Symposium on Photo Interpretation

introduction of infrared photography is particularly useful in combination with
panchromatic recording; this can be obtained either by means of a dual camera
technique or by means of spectrozonal 2-layer films or 3-layer camouflage
detection infrared colour film.
But even in this well known case of chlorophyll infrared reflection there are
controversial opinions as to the usefulness of infrared photography in vegeta
tion surveys. In the U.S.S.R. a large percentage of the aerial photography
concerned is taken on spectrozonal film and has proved to be extremely useful.
In other cases it has been statistically proved that panchromatic photography
is superior to infrared photography for type mapping.
This controversy is caused by a lack of complete understanding of exactly
what happened in the particular airphotographic procedure. This lack is also
demonstrated by the use of vague expressions for assessing the value of one
aerial photograph with respect to another. The conclusions of a comparative in
vestigation of different types of photography are sometimes that one of the types:
- gives a better interpretation than the other-is more useful - shows more clarity
in some features - in German: eines hat eine höhere Aussagekraft als das
andere - die Entschlüsselung wird gesteigert - etc.
Everyone will agree that none of the above judgements expresses exactly what is
good or bad in the tone reproduction densities of the particular aerial negative.
Some time ago, we met with the following case. Aerial photography had
been taken of a large tropical forest area and was delivered to the government
concerned in the form of good quality prints; these were used quite success
fully for a forest inventory. One of the forestry photo interpreters spending his
leave at I.T.C., afterwards showed us the prints and told us that one of the main
problems had been the differentiation between teak and surrounding other
species. We advised him to make a set of special prints on high contrast paper
at an exposure level just sufficient to put the photographic densities of the teak
brightness level on the steep part of the H & D characteristic curve. The re
sulting prints showed a clear differentiation between teak and non-teak, but
were of course heavily under- and over-exposed at the other brightness levels.
Every photogrammetrist and photo interpreter would consider these special
prints as being of very bad quality but they were just right for the particular
purpose and the forester’s comment was, that they would have saved months
of work.
This is a very simple example but, through its simplicity, it demonstrates
how necessary it is that the photo interpreter should realise exactly what he
needs and that he should express this in clear physical-photographic terms in order
that the photographic organization may produce the optimum product for
his requirements.
Image sharpness
Although the expression “sharpness” is not yet well defined, it is generally
understood that it affects the reproduction of the smallest details and edges