Full text: 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

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