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increases from the bottom to the top with a range from bare ground to full green
coverage. Understanding of the right leg in the triangle requires additional study.
It is possibly related to moisture content of ground surface which increases from
top to bottom on the right side of the triangle, corresponding to full canopy, partial
canopy with wet and dry background soils, and open water with on vegetation etc.
The different types of vegetation are related to certain positions within the
triangle. The triangular relationship could be used for broad class vegetation
discrimination and classification (Figure 3). Time sequential comparison of the
triangle models at the same location may be another dimension for change
detection. Any consistent movement of a point is a sign of change. The movement
towards the left of the model means increase in temperature and possibly
decrease in moisture. This is usually a precursor of drought, or plant stress.
Continuous moving towards the desert corner relates desertification process
(Figure 4), Pixels with the same amount of vegetation (50% in Figure 6) could have
a temperature variance of ab, which may be caused by the corresponding
moisture variance be. The Global Triangle Model provides another view angle to
examine the soil background influences on spectral vegetation indices.
Surface temperatures change most widely in non-vegetated areas, the
lowest temperature is normally found in water body at day time, the highest in
desert, the temperatures of wet and dry soils, rocks, and urban areas lay
somewhere in between. In vegetated areas, plants absorb water from soil through
their roots, transport it to leaves, then it evaporates through the leaves. The
évapotranspiration has a cooling effect, which keeps canopy temperature as
constant as possible. This could explain why the temperature varies the most on
the bottom of the model where vegetation is at a minimum, and the temperature
varies little at the top where vegetation is high. The biggest vegetation diversity is
usually found at the high and moderate temperature regions where sufficient
energy is easily obtainable to support photosynthesis process. Millions of species
are actively growing in tropical and temporal regions, forming a complex bio
system. In cooler regions, fewer species are usually found in a less diverse
environment. The moisture variability is the maximum in cool areas, varying
from water body, swamp, and different level of wet to dry soils. In hot areas,
strong evaporation leaves limited or no water on the ground surface. This is why
less and less moisture variance can be observed towards the desert comer of the
triangle. Generally, in fully vegetated areas, temperature is stabilized by plant
évapotranspiration and its variance is minimal, in cool areas, vegetation can not
grow well, and its variance is also minimal, in desert areas, soil moisture is
simply not available and its variance is, again, minimal. The association of the
temperature and moisture variability and vegetation diversity is a theoretical base
to explain why the global view of our Earth in the defined spectral spaces has to be
a triangle.
Studies show that a global triangle shape can be observed in the two
dimensional spectral spaces defined in this study as long as three conditions are
met: (1) a thermal channel and a "vegetation'channel are used to produce the
scattergraph, (2) the image is large enough to include the global features, i.e.
various land cover and vegetation types, and (3) the infrared sensors are
radiometrically calibrated to a range corresponding to the radiance level of plants
and soils.
