from the nadir-direction. The effect of this can be
ing azimuthal an increase up to 25% in the measured radiation in
the infrared band, where the measured radiation in
the visible part of the spectrum is influenced hardly
under most circumstances. It is obvious that this
cients of the effect may not be neglected.
properties in
properties of
of reflection
imation of the
When the
is used, crop
ical behaviour
these errors,
stead of the
ortance of the
uence of the
haviour of the
of possible
Figure 8. Vegetation index VI derived from the data
in figure 7 and the relation between VI and
coverage as computed for the nadir direction.
To cope with this problem, usually the pixel-values
are corrected by means of a quadratic equation. When
crops with different properties show a uniformly
distribution pattern over the observed area, this
correction is shown to work quite well, but when
gradients occur in the observed area, the correction
primarily corrects for the gradient itself rather
than for the differences in radiation caused by the
observation geometry.
ion of cover
ch differ in
eg., the leaf
spherical, the
ed by crop and
ion introduces
e. When an
i platform, the
is 45 degrees
REFERENCES
Bunnik, N.J.J., 1978. The multispectral reflectance
of shortwave radiation by agricultural crops in
relation with their morphological and optical
properties (PhD thesis). Wageningen: Veenman & Zn.
Chen Jialin, 1984. Mathematical analysis and
simulation of crop micrometeorology (PhD thesis).
Wageningen: Agricultural University.
Goudriaan, J., 1977. Crop micrometeorology: a
simulation study (PhD thesis). Wageningen: Pudoc.
Kuebelka, P. & F. Munk, 1931. Ein Beitrag zur Optik
der Farbanstriche. Techn. Physik 11:593-601.
Penning de Vries, F.W.T. & H.H. van Laar (eds.),
1982. Simulation of plant growth and crop
production. Wageningen: Pudoc.
Suits, G.H., 1972. The calculation of the
directional reflectance of a vegetative canopy. R.
S. of Env. 2:117-125.
Wit, C.T., 1965. Photosynthesis of leaf canopies.
Agric. Res. Rep. 663:1-57. Wageningen: Pudoc.
l direction on
•d and infrared
the LAI.
