2. The way of thinking and Method
A histogram of CCT count for band 4 and 5 is synthesized three histograms of three categories
of land area, mangrove forest and waters to compose mangrove area. Generally it has three
peaks but there is often a case with not clear on them. The degree of clear of the peaks based on
the proportion of the sum total each category area, and the existence of land-cover to have
similar spectral reflectance with other category such as shallow or muddy waters, ponds or
swamps in land area, cutovers or new planted areas in mangrove forest and so on. There may be
some a scene without one category. We used band 4 and 5 of TM data of Landsat 5 acquired on
25 April 1990. It covered Mandh area distributed mangrove in the mouth of Inderagiri River,
Sumatera. The histograms of CCT count were shown in the left side of Figure 1.
Figure 1. Histograms for band 4 and 5 and provisional ranges to search threshold (left
side), sB2 of calculated results with Otsu’ method and the maximum (right
side)
Many methods had been suggested for automatic threshold selection in the field of processing to
binary value imagery (Kittler et al., 1986, Ots, 1980, Weszka et al., 1974). In case of simple
histogram of CCT count and to expect enhancive effect, it was reported that several methods
were effective. But these can not be simply applied in case to expect high suitability to the
categories set to put circumstances and convenient for us. We tried a way that threshold was
determined through two steps. The first step was a process to find provisional rage including
threshold on the histogram of CCT count, and the second step was to determine the threshold by
application of fit automatic threshold selection to slightly wide range than the provisional range.
On the histogram of CCT count for band 4 and 5, waters forms shape peak in the lowest range
because waters have lower reflectance than other any categories and the range is very narrow.
The next large and wide base peak composed most pixels of land area and mangrove forest for
band 4. In case of land area and mangrove forest covered with similar component forest on size,
the categories form one peak. In case of land area covered poor vegetation, the category of land
area occupies left side of the peak, and in opposite case the category of mangrove forest
occupies the side. In these case small valley may be formed halfway up or down the peak, but
most pixels occupies overlapped part. For band 5, mangrove forest occupies left side and land
area does right side because this band is sensitive to water content. Between two categories
relatively deep valley is formed because water content of two are quite different. The matters
pointed out above are keys to find threshold between two categories. The provisional rage was
determined to search threshold and pixels corresponded CCT count values in the range were
displayed with white on a black background for each band.
For slightly wide range than the provisional range, Otsu’s method of one automatic selection
method was applied to search threshold. Although the detailed description was omitted, the
calculation was as follows. L is the width of the range. N is total number of pixel in the range
and i n is total number of pixel having CCT count value of i .
N=n1+n2+n3+ . . . +nL ,
pi=ni/N ,
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then, it is required threshold of k which maximizes
sB2.
