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(a) Homogenous forests : Such forests comprise only one 
or a few species and pure stands are common. One or 
two species constitute about 7 0% to 8 0% of Growing 
Stock. Photo-interpretation of such forests is easier as 
identification of species is not a major problem. Coni 
ferous forests of Himalayas, Sal (Shorea robusta) forests 
of Gangetic alluvial plain and Central India fall in this 
group. 
(b) Heterogenous forests : Such forests comprise many 
species, the number may go upto 100 in tropical moist 
deciduous type to nearly 6 00 in tropical wet evergreen 
types. Not all the species are economically important 
and only few species have been recognized on black and 
white paper prints. 
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This type could be further subdivided into two groups, namely 
the Deciduous group and Evergreen group. 
(i) Deciduous group : This type covers a major part of the 
tropical forests and is more amenable to photo interpreta 
tion. The important economic species is Teak which can 
be separated from non-Teak species easily. Generally 
the species classification is made into Teak and non-teak 
type. Measurements of height, crown closure, crown 
diameter, tree count is easier than in the Evergreen group 
and forest type stratification on the above basis can be 
done on medium scale photographs. 
(ii) Evergreen group : It covers about 10% of the tropical 
forest area and is less amenable to photo-interpretation. 
The forests consist of much greater number of species and 
are thick canopied and multi-storied. Identification of top 
canopy species is too difficult and so also the measure 
ment of other photoparameters. Stratification into broad 
classes such as Evergreen, Semi-evergreen is done and 
a large amount of information for forest inventory purposes 
is collected on the ground. The general experience has 
been that leaving aside about 10% of the forest area of 
the country, photo stratification down to detailed level 
could be done on panchromatic aerial photos for forest 
inventory purposes. 
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QUALITATIVE INVESTIGATIONS 
The nature of the forest and the quality of the photographs 
governed by suitable photographic specifications have been mainly inst 
rumental in the extraction of qualitative information. Of the photogra 
phic specifications, season of photography, quality of the camera and 
lens and to some extent scale of photographs have been the chief limit 
ing factors. Significant differences have been noticed in the amount of 
qualitative information obtained with differences in the above specifica 
tions .