Agriculture Reference
In-Depth Information
Table 9.3 Relationships between standard spectral reflectance indices and the nitrogen supply,
expressed by coefficients of determination
Formulas for wavelengths R
(number added is wavel. in nm)
Coefficient of
determination (r 2 )
Definition
Green reflectance
R 550
0.910
Red reflectance
R 670
0.888
Near-infrared reflectance
R 800
0.569
R
R
800
670
Ratio of near-infrared to red
0.911
R
R
800
550
Ratio of near-infrared to green
0.943
R
800
+
R
670
Normalized difference
vegetation index (NDVI)
0.914
R
800
R
670
15 800
.(
R
R
670
)
Soil adjusted vegetation index
(SAVI)
0.907
R
800
+
R
670 05
+
.
2
Red edge inflect. point,
numerical differentiation
dR
d
0.932
=
0
λ
2
Red edge inflect. point, approx.
formula (REIP)
(
R
670
+
R
780
) /
2
R
700
0.970
700 40
+
R
740
R
700
The results are based on natural illumination and vertical view directions (From Heege and Reusch
1996 )
by numerically calculating the second derivative of the reflectance curve and
determining the wavelength where it is zero,
by using empirical approximating formulae.
The disadvantage of the first method is that the reflectance data must be available
with a high spectral resolution. Therefore, many bands are needed and the method
may become expensive. For the second method, several empirical approximating
formulas have been proposed (Dash and Curran 2007 ; Guyot et al. 1988 ). The red
edge inflection point in Table 9.3 , bottom and those in the subsequent text (abbrevi-
ated as REIP ) are based on Guyot et al. 1988 .
The coefficients of determination in the last column of Table 9.3 refer to experiments
with nitrogen fertilization of winter-rye and its spectral results as shown in Fig. 9.19 . All
spectral indices depend on the nitrogen fertilization. However, there are differences. The
800 nm near-infrared reflectance on its own - surprisingly - shows the weakest relation-
ship. The red edge inflection point indices on the other hand are very closely related to
the nitrogen application rate. The best coefficient of determination results from the red
edge inflection point that is obtained by the approximating formula.
Another index that is closely related to the nitrogen supply is the simple ratio of
near-infrared to green (Table 9.3 ). This agrees with results from Solari et al. ( 2008 )
with maize.
The outstanding ability of the red edge inflection point among the various standard
spectral indices for indicating the nitrogen uptake of crops was also demonstrated in
 
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