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Proceedings of the Symposium on Global and Environmental Monitoring

Li Jiang
(Arizona Remote Sensing Center, University of Arizona, Tucson, AZ)
This study is to propose a global triangle model based on the characteristics
of data structure observed in the thermal and NDVI spectral spaces of NOAA-
AVHRR data. The development of the global triangle model is an attempt to study
vegetation in relation to its ecological environment, and to understand the Earth
as an integrated system. —
A scatter plot was produced with data from channel 2 and 4 of the NOAA-
AVHRR scene of January 3, 1989, (Figure 1). Figure 1 is a "global view" of the
West Africa in the near and thermal infrared (NIR and TIR) space. The view
observed looks like a huge triangle containing a variety of plants, soils, and land
surface covers. Similar triangle shape has been observed in five AVHRR data-sets
of West Africa acquired on January 3 and 23, February 11 and 22, and march 23,
1989. This same phenomena was observed when scenes were cut in half but still
contained the various African landscapes. The triangle was also observed using
AVHRR bands 2 and 5 in a similar fashion as observed using bands 2 and 4, since
bands 4 and 5 are both thermal channels performing in adjoining spectral ranges
of 10.5 - 11.5 pm, and 11.5 - 12.5 pm respectively. Four small sites(ocean, bare soil,
dry grassland, and dense vegetation) selected based on visual interpretation of the
image and NDVI value were used to make separate scattergraphs as illustrated
in Figure 1 and 2. Soils occur at the bottom left corner of the triangle where
minimum vegetation and high temperature are usual the case. The vegetation
points lay at the top of the triangle where the pixels are characterized by high
vegetation and relative low temperature. All water pixels crowd around the
bottom right comer of the triangle where temperature and vegetation are both
minimal. If the near-infrared band is replaced with a NDVI band, the
scattergraph still looks like a triangle, but changes in shape and orientation
become apparent. It can be seen from Figure 2, the global triangle is rotated
clockwise in the NDVI and band 4 spectral plane. The high vegetation corner is
tilted towards lower temperature regions, which is often true because of the
cooling effect of green vegetation. The desert corner is lifted a little above the water
comer since some vegetation, more or less, is always found in desert areas. The
basic relationship and distribution of the triangular structure stay the same, and
the thermal and NDVI model looks more reasonable than the triangle observed in
the TIR and NIR space. A possible explanation for this improvement is that the
NDVI is a better vegetation indicator than the near-infrared band alone.
The DN values of deep ocean from the thermal bands were found to be quite
constant both spatially and temporally, and may be used as a reference for
comparison across time and space for vegetation and terrain analysis. DN values
of desert areas vary greatly from day to day and from day to night, (Figure 4 and