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level (which still has to be defined!). Indices such as those suggested in 
this paper could well help city authorities to define limits, restrictions and 
conditions for construction permits. 
An interpretation test was carried out in some parts of the city 
Freiburg i. Br., because both aerial photographs and good statistical data 
were available there. Since some years ago, all statistical publications by 
the city administration of Freiburg show up all data about the economic and 
the population’s structure for larger city-districts, smaller ’’statistical 
districts" and, finally, for very small statistical units, the so-called 
"statistical blocks" (which do not necessarily correspond with normal street 
blocks) (FABRICIUS, 1973). Figure 1 shows a map of a part of the city and how 
it is divided into statistical units. Each of the units is separated from the 
others by streets, streams or other evident boundary-lines. So each unit can 
clearly be identified on the aerial photograph and may be used as an inventory 
unit for photo interpretation. CIR-aerial photography of 1971 and 1972, and 
the results of a census of both population and working places in 1970 (FABRICIUS, 
1973) could be compared directly and without any later correction. The size 
of all statistical units is known and needs not to be measured from the photos. 
Two kinds of information were interpreted from the aerial photos: 
a) the number of trees for each statistical sub-unit as counted under a 
stereoscope 
b) the portion of green vegetation for each statistical sub-unit, acquired 
by counting and classifying dots on a dot-grid. 
There are some remarks necessary to explain the procedure: 
ad a) Resulting from the special functions which trees, especially big trees, 
have to fulfill in an urban environment a separate counting of trees 
was considered to be necessary. These special functions, which result 
from the large surface of big trees and consists of microclimate effects 
such as cooling by shade and transpiration, absorption of dust particles 
and chemicals-and an increase of air moisture, cannot be replaced by 
other, lower forms of vegetation such as turf, gardens and young trees. 
Moreover, the aesthetic effect of trees in a city is completely dif 
ferent from that of turf areas, flower plantations etc. Finally, big 
(= old) trees cannot easily be replaced such as old houses by new build 
ings; if they are removed, this is a decision which cannot be revised 
for a time of at least 20 years. That is why trees in urban areas need 
a special protection and should, therefore, be counted separately from 
other vegetation. 
There is no principal difficulty to count trees from aerial photo 
graphs. Observed under a mirror stereoscope, trees can always easily 
be separated from groups of bushes; also the separation of trees which 
are grown together does not cause many errors. Because of the mentioned 
special ecological functions of big trees and because of the impossibil 
ity of replacing them, it was decided, not to count all visible trees,