Agriculture Reference
In-Depth Information
coeff. of determ. between
N-uptake and wavel. ratio
1000
0.8
r
2
900
0.7
0.6
800
0.5
700
0.4
0.3
600
0.2
0.1
500
0.0
400
400
500
600
700
800
900
1000
wavelength of the numerator in nm
Fig. 9.27
Matrix showing coefficients of determinations (r
2
) of simple reflectance ratios for sens-
ing of N in wheat with a growth-stage in EC or BBCH of 38 (From Reusch
2005
, altered)
The results are unambiguous: the highest coefficients of determination (r
2
) and
hence the most reliable predictions of the nitrogen uptake are obtained with wave-
lengths for both the numerator as well as for the denominator of the index ratio
between about 720 and 800 nm. In the early development stage of EC or BBCH 31
(Meier
2001
), wavelengths somewhat above 800 nm also provided still good results
(Fig.
9.26
). However, in the later stage of EC or BBCH 38 this is not so. At this
stage, the colors in the matrix around 800 nm wavelength already indicate a slight
decrease for the coefficients of determination. Müller et al. (
2008
) as well as Inoue
et al. (
2012
) presented rather similar results of systematic spectral searching in steps
of 10 nm in the form of matrices from experiments with vertical viewing and natural
light respectively for oilseed rape (Brassica napus) in Germany or for rice (Oriza
sativa) in Japan and China.
So the most reliable predictions of the nitrogen uptake come from wavelength
ranges, in which also the red edge inflection point is located (Fig.
9.19
). However,
for reflectance sensing there is a fundamental difference between the red edge
inflection point on the one hand and a simple wavelength ratio from the same spec-
tral range on the other hand. The
red edge inflection point
indicates just a small
dot on the spectral curve that is defined in nm of wavelength and its respective
reflectance. The wavelength ratio instead depends on the slope of the spectral
reflectance curve between the respective wavelengths and is dimensionless. Its pre-
cise definition in terms of wavelengths for the numerator and the denominator
