754 
2.2 Individual Interpretation Keys 
Since the 1960s CIR aerial photography has 
been succesfully applied for the assessment 
and monitoring of vegetation stress and 
disease. Systematic research in the United 
States, Canada, West Germany, Austria and 
France and a number of local tests of the me- 
thode have been promising /7/, /8/, /11/, 
/13/, /20/. 
However, the application of the method to in 
ventory forest decline in larger areas, such 
as ferderal states was far form being satis 
factory. One of the main problem is that 
CIR-film material reveals forest damage 
largely through its source "colour" and that 
it has not yet been possible to develop a 
common standard for the film material itself. 
Consequently the colour varies considerably 
according to emulsion as well as the condi 
tions during aerial photography /4/. Althoug 
the infrared balance can be influenced to 
some degree the criterea colour continues to 
be problematic to differentiate between dif 
ferent damage classes. This is why an indi 
vidual interpretation key had to be estab 
lished for each flight mission. The disadvan 
tages of these "colour" keys became particu 
larly clear during the 1985 inventories taken 
in the Federal Republic of Germany. The CIR 
photographs taken were marked by a striking 
predominance of the colour red due to the 
high relative sensitivity of the infrared 
layer in the emulsion used on these flight 
missions /19/. 
2.3 Universal Interpretation Keys 
Since 1986 a group of experts at the inter 
pretation of CIR photographic evidence of fo 
rest decline from West Germany, Austria, 
Switzerland and Italy (Arbeitsgruppe Forstli- 
cher Luftbildinterpreten, AFL) has joined in 
a team in order to harmonize the photo inter 
pretation procedures and find to standards 
for the assessment of forest damage. One of 
the main goals of the team was to develop 
universal interpretation keys, to minimize 
the relevance of the colour factor as a means 
to differentiate between damage classes and 
to put more emphasis on the crown structure 
in the interpretation of CIR photographs. 
This improvement was facilitated by: 
- better knowledge of those damages in the 
morphology of the crown, which are visible 
in the aerial photography /14/, /15/, /17/ 
and 
- application of special stereoscopes with 
high quality zoom lenses which allow a 
highly differentiated assessment of the 
characteristic 3hape-types. 
Moreover, the use of standard terminology 
and identically structured interpretation 
keys for the entire range of applications is 
meant to ensure that the results of invento 
ries taken on different sites and different 
times can be compared. These are the essen 
tial prerequisites for the use of CIR pictu 
res for the comparison of national federal 
or reginal inventories as well as monitoring 
studies. 
These are the main characteristics of the 
new type of interpretations keys (Table 1), 
/1/: 
- Experts in forestry can make good use of 
them when studying single trees in large- 
scale CIR-photographs (1:3000 - 1:6000). 
- Five classes of "crown condition" have been 
defined according to damage classes estab 
lished in terrestrial inventories. Level 0 
indicates "without visible damage", level 4 
"dead". The interpretation keys for indivi 
dual tree species also distinguish between 
various types of a tree species since the 
appearance of "branche structure" may vary, 
as in case of brush-type is distinct from 
combe-type spruces. The appearance of such 
types varies in aerial photographs too, and 
this is why they are classified separately. 
- The characteristic shape of crowns and 
their reproduction in aerial photography 
are now being used as a criterion for clas 
sification rather than the colouring. Shape