506 
As a principle, always two satellites are 
to be functioning. Hence, everywhere is 
covered four times a day. 
A vegetation index data are made from 
one week data of AVHRR. Daytime data are 
sampled and mosaicked by the lowest 
radiance principle. Cloud covers are 
thus almost eliminated from the results. 
As for the band selection, only visible 
and near infra-red bands are selected. 
Bands 1 and 2 of vegetation index data 
corresponds those of AVHRR bands. Bands 3 
and 4 of the vegetation index data are 
calculated from bands 1 and 2 according to 
the following equations. 
band 3 = band 2 - band 1 + 
band 4 = band 1 - band 2 + c 
band 1 + band 2 
C^,C2 : constant 
3. DATA PROCESSINGS 
One data set was used in this study. They 
are shown in Fig. 1. 
A preliminary classification revealed a 
fairly large shading effect mainly caused 
by sun angle deviations. Those shading 
effects were first eliminated by the 
following equation.(See Fig. 2) 
1' = I/cos[ {7r/2-2tan _1 (r/2R) }- (X ] 
Here, 
1' : image value at point P after the 
elimination of earth curvature 
effects 
I : observed image value at point P 
R : radius of the earth 
(X : declination of the sun 
and 
r = 2Rtan { ( 71/2- (D)/2 } 
where 
(p : latitude of point P 
The classification was done by a maximum 
liklihood method. Training areas were 
chosen according to the World Vegetation 
Map made by Preston James et al. . At 
the first stage, 67 categories were 
selected from this map and spectral 
signatures of the image. After the 
classification, these 67 categories were 
unified to 17 categories as shown in 
Table 1. 
Four kinds of band combinations, i.e. 
band 1 and 2, band 3 and 4 , band 1,2 and 
4, and all 4 bands, were tried. From 
these results, the original two bands 
combination shows a good discrimination 
between forests and crops while the 
vegitation index bands show a relatively 
comprehensive discrimination. 
From the purpose of this study, the 4 
bands combination was selected for further 
processings. The classified results were 
converted to longitude and latitude 
coordinates as shown in Fig.3 and the 
acreages of each categories were 
calculated as shown in Table 2. 
4. CONCLUSION 
As a result of this study, the following 
conclusion were derived. 
1. The NOAA vegetation index data were 
proved to be a suitable data for 
world wide vegetation monitorings. 
2. A method for the vegetation 
classification of NOAA vegetation 
index data was proposed. 
3. Though vegetation index data are 
nearly cloud-free, remaining clouds 
were still problems for classi 
fication . 
4. As the actual ground truth is almost 
impossible for this kind of studies, 
more reliable sources for world 
vegetation distributions are 
necessary. 
5. REFERENCES 
1) 'World Vegetation Map', 
Preston E James et al, 
The TIMES Atlas of the World, TIMES 
Books, London, 1981.