examples of linear combination indices of 2 or more spectral bands that have been
applied to Landsat imagery and coarse resolution Moderate Resolution Imaging
Spectroradiometer (MODIS) (Justice et al. 1998; Crist and Cicone 1984; Lobser
and Cohen 2007). These indices have the added advantage of generating multiple
measures of subpixel components, including vegetation, soil, shade, and other
scene elements (Souza et al. 2003). The shade fractional component in SMA
provides a useful measure of canopy shadowing that has been used to diagnose
tropical forest structural properties (Anderson et al. 2011).
Linear combination spectral indices form parallel isolines of constant slope and
increasing NIR intercepts, in Red-NIR space, significantly departing from how
ratio-based and normalized difference VIs characterize vegetation spectral variations in time and space (Fig. 1.3a). Soil and vegetation are modeled as non-interacting horizontal, or checkerboard, fractions within a pixel, and the vegetation
fraction is retrieved using linear mixing theory. The shade fraction in SMA is
sometimes used to account for non-linear mixing of soil and vegetation components
as expressed by canopy shading from vertical tree and shrub canopy structures.
In the case of optimized indices, simplified radiative transfer theory on soilvegetation interactions is utilized. This involves formulating a VI equation so that
the VI isolines line up with the vegetation biophysical isolines (Figs. 1.3, 1.4). The
enhanced vegetation index (EVI) gains its heritage from the soil-adjusted vegetation index (SAVI) and the atmosphere resistance vegetation index (ARVI, Kaufman and Tanre 1992), and is an optimized combination of blue, red and NIR
bands, designed to extract canopy greenness, independent of the underlying soil
background and atmospheric aerosol variations,
SAVI ¼ 1 þ L
ð
Þ q NIR Àq Red
ð
Þ= L þ q NIR þ q Red
ð
Þ
ð 1:7Þ
EVI ¼ 2:5 q NIR Àq Red
ð
Þ= L þ q NIR þ C 1 q Red À C 2 q Blue
ð
Þ ;
ð1:8Þ
where q are reflectances in the NIR, Red, and Blue bands, respectively; L is the
canopy background adjustment factor; and C 1 and C 2 are the aerosol resistance
Fig. 1.4 Illustration of first order two layer canopy model of canopy layer and background used
in theoretical studies of isoline patterns and interactions with atmosphere, sensor-canopy-sun
geometry, and in formulating optimized vegetation indices. Ó [2000] Adapted and reprinted with
permission from Yoshioka et al. (2000)
1 Indices of Vegetation Activity
7
applied to Landsat imagery and coarse resolution Moderate Resolution Imaging
Spectroradiometer (MODIS) (Justice et al. 1998; Crist and Cicone 1984; Lobser
and Cohen 2007). These indices have the added advantage of generating multiple
measures of subpixel components, including vegetation, soil, shade, and other
scene elements (Souza et al. 2003). The shade fractional component in SMA
provides a useful measure of canopy shadowing that has been used to diagnose
tropical forest structural properties (Anderson et al. 2011).
Linear combination spectral indices form parallel isolines of constant slope and
increasing NIR intercepts, in Red-NIR space, significantly departing from how
ratio-based and normalized difference VIs characterize vegetation spectral variations in time and space (Fig. 1.3a). Soil and vegetation are modeled as non-interacting horizontal, or checkerboard, fractions within a pixel, and the vegetation
fraction is retrieved using linear mixing theory. The shade fraction in SMA is
sometimes used to account for non-linear mixing of soil and vegetation components
as expressed by canopy shading from vertical tree and shrub canopy structures.
In the case of optimized indices, simplified radiative transfer theory on soilvegetation interactions is utilized. This involves formulating a VI equation so that
the VI isolines line up with the vegetation biophysical isolines (Figs. 1.3, 1.4). The
enhanced vegetation index (EVI) gains its heritage from the soil-adjusted vegetation index (SAVI) and the atmosphere resistance vegetation index (ARVI, Kaufman and Tanre 1992), and is an optimized combination of blue, red and NIR
bands, designed to extract canopy greenness, independent of the underlying soil
background and atmospheric aerosol variations,
SAVI ¼ 1 þ L
ð
Þ q NIR Àq Red
ð
Þ= L þ q NIR þ q Red
ð
Þ
ð 1:7Þ
EVI ¼ 2:5 q NIR Àq Red
ð
Þ= L þ q NIR þ C 1 q Red À C 2 q Blue
ð
Þ ;
ð1:8Þ
where q are reflectances in the NIR, Red, and Blue bands, respectively; L is the
canopy background adjustment factor; and C 1 and C 2 are the aerosol resistance
Fig. 1.4 Illustration of first order two layer canopy model of canopy layer and background used
in theoretical studies of isoline patterns and interactions with atmosphere, sensor-canopy-sun
geometry, and in formulating optimized vegetation indices. Ó [2000] Adapted and reprinted with
permission from Yoshioka et al. (2000)
1 Indices of Vegetation Activity
7