Agriculture Reference
In-Depth Information
Fig. 9.25
Control of nitrogen spreading via reflectance sensing based on artificial light with
oblique viewing of the canopy from the roof of the tractor (From Agri Con GmbH, Jahna, Germany)
In short, sensing by oblique viewing of the canopy particularly needs reliable
reflectance indices that do not saturate. This is dealt with in the next section.
9.4.3.1
New Reflectance Indices Instead of Standard Indices
The standard spectral indices dealt with so far mostly have been developed for
remote sensing of vegetation and landscapes from satellites and aerial platforms.
The fact that some of these indices depend on the nitrogen supply of crops does not
imply that they are best suited for the detection of this nutrient. The “Normalized
Difference Vegetation Index” (NDVI) is widely used as the standard index for
assessing the “greening” of the global surface. However, despite this, the potential
of the NDVI to indicate the density or the leaf-area-index of the vegetation is rather
limited. Therefore Schepers et al. (
1998
) proposed to use a
green NDVI
instead of
the
standard NDVI
. In this green NDVI, the red reflectance in the standard NDVI
equation (Table
9.3
) is substituted by green reflectance.
It is well known that the standard NDVI saturates around a leaf area index of
that is three to four times higher. A similar saturation effect applies to standard
infrared to red ratios. The reason for this is the very high absorption of red light by
photosynthesis. Because of this, the light does not penetrate deeply into dense crop
canopies and hence is not able to sense higher leaf-area-indices. In the green and in
