REPORT OF COMMISSION VII 
PAL Ms 
SS 3 X 
       
S334] saask 
INSTRUCTIONS 
1. Set pointer of each of the 2 inner wheels opposite point labeled “START” on the outer 
wheel. 
2. Examine aerial photos and decide whether area in question is well-drained or poorly- 
drained. This determines which of the 2 pointers should be rotated until it points to the pertinent 
combination of photo image characteristics written on the outer wheel. The other pointer should 
remain at the “START” position. 
3. When the area in question has been interpreted in terms of all pertinent features listed on 
the outer wheel and the pointer has been rotated to the corresponding position, the correct vegeta- 
tion type can be read through one of the windows. 
  
FrG. 7. Portion of a disc type of photo-interpretation key (top two discs removed) 
together with instructions for its use. 
particular group: (1) elimination keys are preferable to selective keys for teach- 
ing photo-interpreters how to distinguish systematically one vegetation type 
from all others, (2) selective-keys, particularly those of the integrated-selective 
type, constitute very helpful supplements to the elimination keys, (3) disc 
and punch card keys, because of their space limitations, do not permit as complete 
a description of each class as does the dichotomous key, (4) the dichotomous key 
necessitates the interpretation of recognition features in a prescribed sequence 
which in some cases is disadvantageous. Thus, as seen in Figure 9, if the inter-