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Title
Remote sensing for resources development and environmental management
Author
Damen, M. C. J.

228
sured. The subjective and qualitative nature of the
first approach and the destructive sampling of a
few plants, resulting in a large variability be
tween measurements, of the second approach are the
main disadvantages.
Remote sensing techniques provide quantitative in
formation about a whole field trial instantaneously
and non-destructively. The aim of the research of
this paper was to establish whether remote sensing
can support and/or replace conventional field mea
surements in field trials. Emphasis was on impro
ving precision in field trials.
2 METHODOLOGY
2.1 Aerial photography
Black and white multispectral aerial photography
was carried out by taking vertical photographs from
a single-engine aircraft, equipped with Hasselblad
cameras with Planar 100 mm lenses, at intervals of
about two weeks. A Minolta photometer was used for
correcting the camera settings for differences in
illumination between missions, and the expected
reflectance level of the objects was also taken
into account for obtaining correct levels of ex
posure.
Missions were only carried out under nearly con
stant atmospheric conditions, and during a mission
camera settings (exposure time and relative aper
ture) were not altered for a certain film/filter
combination. By a judicious choice of aerial films
with satisfactory sensitivity used in combination
with filters, the optimal wavelengths identified by
Bunnik (1978) were matched: a green band (555-580 nm),
a red band (665-700 nm) and an infrared band (840-
900 nm) (see Clevers, 1986a). The development of the
films was standardized.
Densities were measured by means of an automa
tized Macbeth TD-504 densitometer with an aperture
of 0.25 mm. After converting the densities to radi
ant energy values by means of the characteristic
curve, compensation for off-axis imagery was made
for each image point (only for light fall-off; vig
netting was avoided). There exists a linear rela
tionship between this corrected radiant energy value
and the reflectance of the object (Clevers, 1986a,
1986b). In this linear function the exposure time,
relative aperture, transmittance of the optical
system, irradiance, path radiance and atmospheric
attenuation were incorporated. Reference targets
with known reflectance characteristics were set up
in the field during missions and recorded at the
same camera setting and under the same atmospheric
conditions as the field trials, in order to ascer
tain the parameters of the linear function.
Reflectance factors ascertained in this way with
multispectral aerial photography appear to be well
calibrated when compared with MSS and radiometers
in the field (see Clevers, 1986b), supplying infor
mation that is only influenced by the object. This
allows multitemporal comparison and analysis.
2.2 Test area
The research was carried out at the ir. A.P. Minder-
houdhoeve, experimental farm of the Wageningen Agri
cultural University (The Netherlands), situated in
one of the new polders, Oost-Flevoland, which was
reclaimed about 30 years ago. The new polders are
flat, uniform and highly productive agricultural
lands with a loamy topsoil.
2.3 Experiments
The usefulness of remote sensing techniques in agri
cultural research was verified by means of analysing
several field trials.
Field trial 116 in 1982: Barley in a split-plot de
sign in three replicates (cultivar "Trumpf"). Whole-
plot treatments were 2 sowing dates: 26 March (Zl)
and 22 April (Z2). Split-plot treatments were 6 ran
domized nitrogen levels: 0-20-40-60-80-100 kg N per
ha (N1 to N6).
Field trial 116 in 1983: Barley in a split-plot de
sign in four replicates (cultivar "Trumpf"). Whole-
plot treatments were 2 sowing dates: 7 March (Zl)
and 21 April (Z2). Split-plot treatments were 5 ran
domized nitrogen levels, applied as split dressing
(in kg N per ha):
before sowing
Feekes stage 7
Total
N1
0
0
0
N2
20
0
20
N3
20
20
40
N4
20
40
60
N5
20
60
80
For Feekes stages see Large (1954).
Field trial 92 in 1982: Spring wheat in a split-plot
design in three replicates (cultivar "Bastion", so
wing date: 26 March 1982). Whole-plot treatments
were 2 fungicide treatments: no fungicides at all
(F0) and 4 kg Bavistin M per ha at Feekes stage
5 combined with 0,5 1 Bayleton per ha at stage 10.4
(FI). .Split-plot treatments were 2 sowing densities
and 4 nitrogen levels, which were completely random
ized within the whole plots. The sowing densities
were: 150 seeds per m * 2 3 (SI) and 300 seeds per m 2
(S2). Only results for one nitrogen level will be
presented (20 kg N per ha before sowing combined
with 80 kg N per ha at F8).
Field trial 92 in 1983: Winter wheat in a split-plot
design in three replicates (cultivar "Arminda", so
wing date: 25 October 1982). Whole-plot treatments
were 2 fungicide treatments: no fungicides at all
(F0) and 3 kg Bavistin M per ha at F5 combined with
2 kg Bayleton CF per ha at F10.4 (FI). Split-plot
treatments were 3 sowing densities and 4 nitrogen
levels, which were completely randomized within the
whole plots. The sowing densities were: 150 seeds
per m 2 (SI), 300 seeds per m 2 (S2) and 600 seeds
per m 2 (S3). Only results for one nitrogen level
will be presented (120 kg N per ha at F8).
Field trial 95 in 1983: This trial was similar to
field trial 92 in 1983. The cultivar of winter wheat
was "Okapi" and the sowing densities were: 100 seeds
per m 2 (SI), 200 seeds per m 2 (S2) and 400 seeds per
m 2 (S3). Only results for one nitrogen level will be
presented (40 kg N per ha before sowing combined
with 100 kg N per ha at F8).
3 RESULTS
3.1 Spectral Reflectances
In field trial 116 in 1982 reflectance factors were
ascertained by means of multispectral aerial photo
graphy in green, red and infrared spectral bands.
The effects for two nitrogen levels for two sowing
dates are illustrated in figure 2 for all three
bands. In this figure the reflectance is plotted
as a function of days after sowing. The patterns
of green and red reflectances were similar. The re
flectance in the visible bands decreased with in
creasing growth during the beginning of the growing
season. On the first recording date (28 April) only
bare soil was present, resulting in similar reflec
tance values in all spectral bands considered. At
complete soil cover the reflectance in the visible
bands remained fairly constant. At the end of the
growing season this reflectance increased due to
senescence of the crop. The sowing date effect was
evident. The late-sown crop showed a faster develop
ment due to higher temperatures during germination
and initial growth than the early-sown crop, re
sulting in a shorter growing season (shift in re
sponse). There were significant (negative) nitrogen
effects on reflectances in the visible bands, but
the differences were small. For the infrared reflec-
. a
GREEN RE
b
RED REFL
20
15
10
0
0
c
INFRARED
60
50
40
30
20
10
Z2
0
0
Figure 2
(c) infi
and Z2)
Field tr
tance th
was evic
red spe(
visible
trogen i
throughc
In fie
two nitr
trated j
sowing c
was a ne
(mostly
season t
frared r
dates. I
less prc
In adc
soil mo]
son in
moisture
all thre
tively j
top soil
the earl
(for the
tained c
after s<
spective