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(721) 
THE PREPARATION OF PHOTO-INTERPRETATION KEYS 
by 
Hugh T. O'Neill. 
A few years ago a Lt. Commander of the Navy, who survived most of 
the terrible battles of the Pacific, impressed by the sweeping claims of some 
photo-interpreters for finding oil, gold, diamonds, etc., asked to have a key 
made by which from vertical aerial observation or air photography of his 
roof, he could deduce if it was safe for him to enter his house, 1. e. if, in this 
way, he could ascertain in what humor his wife was. 
In contrast to (and probably because of) the extravagant claims of some 
photo-interpreters, there is a sweeping obdurate skepticism that doubts nearly 
all claims made by photo-interpreters. For example, the following five fre- 
quently repeated statements may be cited: 
1. No two photo-interpreters agree in their identification of objects on 
small scale photographs. 
2. Vegetation is too unreliable as an indicator of environment. Forest 
fires, lumbering, floods, clearing, etc., can obliterate the vegetation at any 
time and the succeeding vegetation is even more unreliable. 
3. Only a very specialized botanist with a life-time of field study can 
recognize individual trees on air photographs and then only very doubtfully. 
4. It is impossible to identify individual trees on vertical photographs 
except on very large scales, e. g. 1/500 to 1/1,000. 
5. Further, seasonal coloration of trees is of no practical use in identifying 
them on color or black and white photography because the coloration is far 
too variable from year to year for individuals of each species. 
It is hoped to show here that all five of these statements are partly or 
entirely wrong and that precise well-illustrated keys are a very important 
means by which the photo-interpreter can avoid the errors upon which these 
five objections to photo-interpretation are commonly and loosely made. 
Now, What is a Key? In its broadest sense, a Key is a systematic procedure 
using contrasting characters for the identification of one object from another 
object. To this definition must be added for our purposes, “on photography”. 
A practical example should make this meaning clearer. A lumber and milling 
company in the Carolinas, manufacturing spools, bobbins and spindles out of 
dogwood, wants to know where dogwood can be found and in what quantity. 
For this purpose, I submit the following key (and accompanying procedure): 
Take air photographs on a scale of 1/40,000 on panchromatic film of ordinary 
grain size (e. g. Super XX of Eastman). Neither infra-red nor color film is as 
good as panchromatic for this purpose. The pictures must be taken when 
dogwood is in bloom, i. e. just after the apple-orchards in the immediate 
vicinity have finished blooming; no other time of the year will serve this 
purpose. On such photography the dogwood can be identified by means of the 
following key by inexperienced persons thus refuting four out of five of the 
objections above listed. Anyone in the audience can verify this for himself 
by examining the aerial photographs on display here (These photographs are 
included here as Figure 1). Pure white dots present in the vegetation in the 
55