166
Percent of
1954
1980
1994
forest pixels > threshold value
2.49
15.38
1.17
non forest pixeis
2.87
17.07
7.94
threshold value
150
100
185
Tab. 1 Forest area changes and threshold values
3.2 Homogeneization
A more sophisticated analysis has been carried out to
avoid the problems above. The local gray level histogram
has been compared with the histogram of the forest
sample area, in each area the percentage of pixel which
would be classified as forest with the previous scheme
must be higher than the percentage in the sample forest
area.
A 5x5 pixels mask has been applied to the bina-7 map
and the density of “forest” pixel has been evaluated. The
central pixel has been classified as forest if the “forest”
pixel density is equal or greater than the density in the
sample forest area.
A further step of the elaboration involves a mask similar to
the previous to obtain more homogeneous forest regions.
A final comparison has been carried out to verify the
correspondence between the new classification and the
“ground truth” of the ortho-photo (Fig. 12 and 13).
Fig. 11 Step 1: binary map overlaid to the ortho-photo
3.3 Time comparison
A time-wise comparison has been carried out on the
binary maps representing the forest coverage at different
years. This allows to discriminate the forest coverage
change over time, both in a qualitative way and in a
quantitative way.
Before starting the analysis the binary map have been
split in two different regions to separate the higher part
(more than 1000 meter above the sea level) where the
forest coverage is more continuous, and the lower part
where the presence of little farms and houses makes the
forest coverage more leopard-skin looking like (between
400 and 1000 meter above the sea level).
Fig. 12 Step 1: binary map overlaid to the ortho-photo
A growth trend is clearly visible in the tables below. The
Fig. 13 Forest area coverage in 1954
higher areas show the strongest trend. The situation in the
lower areas is complex and both expansion and recede
areas are present.
Total area in square km
11.57
From 1954 to 1983 [%]
67.60
From 1954 to 1983 [Square km]
2.3
From 1983 to 1994 [%]
68.30
From 1983 to 1994 [Square km]
0.79
From 1954 to 1994 [%]
67.90
From 1954 to 1994 [Square km]
3.09
Tab. 2 Forest growth for areas higher than 1000 m above the
sea level.
Total area in square km
15.62
From 1954 to 1983 [%]
32.99
From 1954 to 1983 [Square km]
1.1
From 1983 to 1994 [%]
31.15
From 1983 to 1994 [Square km]
0.36
From 1954 to 1994 [%]
32.08
From 1954 to 1994 [Square km]
1.46
Tab. 3 Forest growth for areas lower than 1000 m above the
sea level
