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TECHNIQUES OF PHOTO-INTERPRETATION 
by 
C. A. J. von Frijtag Drabbe, 
Director of the Dutch Topographic Service. 
I am expected to give you my ideas about techniques of photo-interpre- 
tation. It has become clearer to me from the meetings during this world Con- 
gress why there is a marked difference between the significance of the word 
*Photo-Interpretation" in the old and in the new world. 
I thought that in the New World only another separation is made in the 
whole field of the use of aerial photographs, thus bringing the use of these 
photographs for ordinary mapping purposes into the domain of photo-inter- 
pretation whereas in the old world generally this is not done. I did not quite 
understand why — whereas from a topographical point of view — an aerial 
photograph is nothing but an overly detailed map — people could call this 
reading and measuring of aerial photographs photo-interpretation. 
For I think there is no one in Europe who calls reading of a topographical 
map *map interpretation". 
It is only in tactics and strategy that we can deduce from the whole shape 
of an aera as it is shown on a map, the value of that area for attack or defence 
or whatever it may be. This we might perhaps call tactical or strategical map- 
interpretation. Now, the same thing holds for aerial photographs. 
Reading topographical data from aerial photographs, such as roads, rail- 
roads, highways, dikes, woods, pastures, rivers, canals, buildings, etc. has noth- 
ing to do with photo-interpretation. This is just photo-reading. 
But during a lecture I held at A.U.S. about photo-interpretation I was 
asked whether I could bring all my interpretation data in their proper place 
on the maps. As I answered "Yes, of course" the officer who asked me that 
question said *Then you are a lucky man for too often we cannot because the 
maps are inadequate or do not exist at all.” 
Furthermore, one of the speakers at the Congress told us that he planned 
more than 1000 miles of a road project on air photographs, maps not being 
available. Hence it might be that in America one speaks of photo-interpreta- 
tion because one has to use his photographs as a map. As he is not a topo- 
grapher he has really to “interpret” himself those topographical data, and thus 
the separation between photo-interpretation and photoreading becomes more 
or less vague. 
The vastness of the United States and the relatively small population make 
it comprehensible that not all data from aerial photographs have been trans- 
lated into topographical maps. I should like to make this quite clear. This may 
also be the reason why in America there is attached so much importance to 
photo-interpretation keys. In order not to be misunderstood I wish to state 
that I realize the great value of these interpretation keys. 
The oil geologist and the forester among others will immediately see the 
big value of photo interpretation keys as they achieve their purpose much