165 
Fig. 10 Detail of the 1954 Ortho-photo 
3.1 Histogram interpretation 
The first step of the image classification is the 
construction of the gray level distribution histogram. On 
each image two test areas which represent a forested 
area and a non forested area respectively have been 
Forested-non forested area in 1983 
Gray level histogram 
Fig. 8 Gray level histogram for test areas in 1983 
chosen. 
A first classification scheme simply uses a threshold 
value to assign a pixel to a forest area rather than to a 
non-forest area according to its gray level value. 
Binary maps for forested-non forested area have been 
obtained in this way. 
The threshold values have been chosen for each image 
by making minimum the error of attributing a pixel to the 
Forested-non forested area 1994 
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Fig. 9 Gray level histogram for test areas in 1994 
wrong class. 
Some problems arise using this classification technique: 
high gray level values due to noise in forest areas result in 
a leopard-skin pattern binary map and areas with dark 
shadows are classified as forested regardless to their real 
coverage. 
This can be easily verified by superimposing the obtained 
binary forest maps on the ortho-photos (Fig. 11). 
the gray level values using the image analysis capability 
of Grass G!S. To get rid of gray level noise peaks, a low 
band pass filter has been applied to the original images. 
Using this method we are able to realize maps of the 
Fig. 5 Orthophoto of the year 1983 
extension of the vegetation in each photograph and to 
compare them through the years. 
Fig. 6 Orthophoto of the year 1994 
3. IMAGE ELABORATION 
The main step of this procedure is the classification of the 
images. This is a well known problem in the case of multi- 
spectral image analysis (Jensen, 1986) but no algorithm 
can be found in the literature for panchromatic images 
(images with only one band). Therefore the classification 
that will be presented in the next paragraph is original. 
Forested-non forested area in 1954 
Fig. 7 Gray level histogram for test areas in 1954 
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Gray level histogram 
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