Description of fieldwork site and collected data
Barton Bendish Farm, Norfolk, 1997
The data used in this experiment were collected during the growing season of
1997 from a study site based around Barton Bendish Farm, Norfolk (LAT, LONG). The
farm covers some XX hectares and produces winter wheat, several varieties of spring
barley, and sugar beet for commercial sugar production.
Three barley fields, two sugar beet fields and one wheat field were selected at the
start of the fieldwork campaign. Figures X.1a and X.1b show the fieldwork site at two
Figure X.1a SPOT-XS image of Barton Bendish fieldwork site
(22/07/97).
different scales. Figure X.1a shows a SPOT-XS (acronym??) image of the fieldwork site
and the surrounding areas. Figure X.1b shows an aerial photograph of the site obtained
by the Natural Environmental Research Council (NERC) Airborne Research System (??)
(ARS) aircraft on XX, 1997. In both cases, the fields selected for the study are marked. It
solar zenith
angle
(assuming
parallel rays)
rotating boom
radiometer
head
azimuth angle
viewing direction
view zenith
angle (+ve)
shadow of boom and
radiometer head
can be seen that there are quite large variations in the soil brightness throughout the site,
caused by differences in soil composition and drainage efficiency. To compensate as far
as possible for any effect this variability may have on crop density and/or structure, as
many samples as time allowed were taken from within each field. Data were also
collected from as many of the fields as possible during each visit to minimise the
possibility of selecting an atypical field of any particular crop.
It was intended that extended visits would be made to the field site
approximately every two to three weeks throughout the spring and summer in order to
provide detailed temporal samples throughout each stage of crop development. Due to
the appalling early summer weather however, this was not possible. There were very few
dry days during May and June, and even fewer days of clear sunshine. As a
consequence, there are undesirable gaps in the data, which means the temporal sampling
is not as complete as was initially planned.
Summary of collected data
The field data collected on N visits to the Barton Bendish site between April
N, and August N, 1997, consist of:
 ground spectro-radiometric measurements – samples of directional
reflectance were made with a boom-mounted PS-II radiometer in the visible to nir
wavelength range (due to limitations in the instrument, effectively, 450-950 nm).
Measurements were made at 10o intervals from –70o to +70o degrees in both the
principal and cross-principal solar planes1. The viewing and illumination geometry
are illustrated in fig. XX. In addition to the target radiance measurements, nearsimultaneous radiance measurements of a polythene (assumed) Lambertian reference
panel were made. Reflectance was readily calculated as the ratio of the radiance of the
target to that of the reference panel (the definition of BRDF – see section XX). In
addition, for each reference panel measurement that was recorded, the so-called dark
1
Negative angles in this case imply measurements in the backscatter direction, and
positive ones the specular direction.
Figure XX. The viewing and illumination geometry of the radiometer measurements
current response of the instrument was also recorded. This is a measurement of the
signal recorded by the radiometer when no light is incident on the detector, caused by
background thermal excitiation of the silicon detector. The dark current is subtracted
from the measured radiance to provide an accurate measurement of target radiance.
 canopy coverage estimates - nadir-pointing photographs were taken of the
various canopies throughout development. The photographs were processed into a
digital format (Kodak Photo-CD (PCD) details ??) and used to estimate canopy
coverage. This was done by estimating the proportion of visible soil per unit ground
area (Disney et al., 1998). Full details of the processing are described in section XX.
 gap probability - a limited number of nadir-pointing hemispherical
photographs were obtained. If the camera lens geometry is known, a map of the
angular variation across the scanned images can be calculated. An estimate of the
joint gap probability through the canopy can then be made by classifying the images
into the visible soil and canopy elements, and relating the visible soil areas to view
zenith angle across the images.
LAI / leaf angle distribution measurements – LAI measurements were made
with a LiCor LAI 2000 instrument. The instrument works by comparing the intensity
of incident radiance measured at the bottom of the canopy with that arriving at the top
(LI-COR Inc. Techincal report). (APPENDIX?) Four assumptions are made regarding
the measured canopy: (i) the vegetation within the field-of-view of the instrument is
assumed to be optically black i.e. no reflectance or transmittance occurs through the
leaves (ii) foliage elements are small compared to the area of view for each ring (iii)
the foliage is randomly distributed within certain foliage-containing envelopes (iv)
foliage is azimuthally randomly orientated. The field-of-view of the instrument is
close to hemispherical. Light
incident on the instrument lens is recorded by a
photodiode detector divided into 5 concentric angular rings, each of approximately
15o in width. The gap fraction at zenith angle , T(), is calculated as
D 
 T  
D  
Equation X.1
where D() is the diffuse intensity below the canopy at view angle , and D’() is the
diffuse intensity above the canopy at view angle . T() determines the probability of
non-interceptance and depends on the leaf normal orientation, the leaf area density
and the pathlength through the canopy (Ross, 1981; another ref.?). T() strictly is also
a function of the view azimuth angle, , but as the instrument assumes azimuthal
homogeneity within the canopy this dependence is ignored. This is directly analogous
T    exp  G S  
Equation X.2
to light transmittance through a scattering liquid, which is controlled by the extinction
coefficient, concentration of scatterers and pathlength, and is described by the BeerLambert law
or
where G() is the leaf projection function (Ross, 1981) i.e. the fraction of foliage
 ln T    G S  
Equation X.3
projected towards angle ;  is the leaf area density (i.e. the one-sided leaf area per
unit volume of canopy in m2m-3); S() is the path length through the canopy at angle
, which, if the canopy height is z, must be z/cos.  can be calculated as (Miller,

  2 2
0
ln T  
sin d
S  
Equation X.4
1967)
-lnT()/S() is known as the contact frequency, and represents the average number of
contacts a probe passing through the canopy at zenith angle  would make with
foliage elements, per m. If the canopy is of height z, the pathlength S() at zenith
angle  is then z/cos(), and the LAI is the leaf area density multiplied by the height
of the canopy i.e. z. In this case,

LAI  2 2 ln T   cos sin d
Equation X.5
0
and this integration is approximated as a summation over the five angular rings of the
instrument, with the sind factor replaced by the calculation of a constant weighting
factor w() for each ring due to the fixed geometry i.e.
5
LAI  2 ln T  i  cos  sin w i 
Equation X.6
i 1
The
manufacturers
recommendations
were
followed
in
deciding a
measurement plan. One measurement was made at the top of the canopy followed by
ten measurements at different locations below the canopy. This pattern was repeated
three times, and the resulting 33 samples comprises one full set of measurements.
Care was also taken over the spatial distribution of the measurements within each set.
In order to avoid sampling problems caused by the strongly row-oriented nature of the
crops, measurements were made following transects at a diagonal angle of between
30o and 45o to the row direction with sufficient spacing between each measurement to
avoid measuring at integer numbers of rows or inter-row gaps. If care is not taken in
this way an aliasing effect can occur, for example by always measuring between or
within the vegetation rows, with a consequent under- or over-estimation of LAI. In
addition, care was taken with regard to the illumination conditions under which
measurements were made. The instrument operates under the assumption that the
canopy is only illuminated by a diffuse source, and therefore should be shadowed
from any direct sunlight. Due to the weather during the fieldwork campaign it was
possible to make the majority of the measurements under diffuse conditions. On days
where direct sunlight was in evidence, a 270o view cap was used to mask out both the
operator and sun from the viewing hemisphere (i.e. measurements are always made
with the sun behind the observer with the cap obscuring the 90o sector centred at on
the observers position). In addition, on sunny days measurements were made as late
or early as possible in the day when penetration of the canopy by direct sunlight is far
lower due to the much higher solar zenith angles.
 plant structural measurements - detailed descriptions of the topology and
structure of individual plants were made for use within the BPMS model (Lewis,
1996; Lewis and Disney, 1997). Typically three to five plants per field per date were
measured. A set of N measurements were made of each plant. Details of the
measurements are as follows:
For each tiller:
tiller azimuth angle
tiller zenith angle
For each stem section (emergent from ground, and between leaf nodes):
stem length
stem width
For each leaf:
leaf base zenith angle
leaf tip zenith angle
leaf length
% leaf length to maximum width
leaf width
Individual leaf angles (base, tip) and stem section angles were measured
manually with a protractor to an estimated precision of 5 degrees. Leaf length,
maximum width and stem lengths were measured manually, to an estimated precision
of 1 mm. Figure X.1 shows an example of a barley plant measured in the field, and
illustrates the measurements described above.
stem zenith
angle
leaf tip
zenith angle
leaf length
length to
maximum
width
stem length
Figure X.1 Wireframe representation of a young barley plant (15/05/97) showing the
measured plant parameters.
 Crop height – multiple manual measurements of canopy height were made for all
canopies on all dates.
 Plant and row spacings – row spacings in each field were recorded along with the
planting densities of the various crops2.
 Meteorological data – estimates of atmospheric temperature and pressure in
addition to ground level visibility estimates are available from Brooms Barn
Experimental station.
 Crop yield data – yield data are available on a per-field basis for 1997 and the
preceding years from the farm manager.
 Airborne data - consisting of:
 Daedalus 1268 Airborne Thematic Mapper data (ATM) – radiometer
measurements in 11 broadband channels in the visible/NIR, SWIR, and TIR. Parallel
flightlines were flown over the scene to provide overlap which gives BRDF sampling
in the central region of overlap due to the wide FOV of the sensor (Barnsley et al.,
1997). The data were recorded at an altitude of 3000m giving a spatial resolution of
around 5m. Forty three flightlines were flown in total, on four dates: 22/4/97, 4/6/97,
5/6/97 and 6/8/97. It should be noted that the data from 22/4 and 4/6 suffer from haze
problems in some areas (5/6/97 date was flown as a contingency to cover the
eventuality that the data from the previous day might be unusable).
 CASI (XX) (spectral mode) hyperspectral radiometer data, consisting of
288 channels in the visble/NIR, obtained concurrently with the ATM data. Forty two
flightlines were flown in total, on the dates given above, with a spatial resolution of
approximately XX m.
 Aerial photography (false colour) obtained concurrently with the ATM and
CASI data. The resolution of the photography is approximately XX m.
2
This does not take account of some areas in the barley and wheat fields where the seed drilling
vehicle has crossed back on itself leading to patches of doubly drilled crop. These areas are noticeably
denser than the main body of the fields and were avoided for all measurements.
 Satellite data - consisting of:
 ERS-2 PRI data were obtained for 14/3/97, 18/4/97, 23/5/97, 27/6/97 and
1/8/97. Some problems were encountered with conteporaneous ground measuremnts
for the last few dates due to heavy rainfall.
 SPOT XS data were obtained for 4/4/97, 11/7/97 and 22/7/97. Some cloud
and haze are present in parts of the 11/7 and 22/7 scenes.
 NOAA AVHRR data - daily AVHRR data for East Anglia (down to London)
from January 1997 to August 1997 were obtained.
 POLDER data - not yet obtained/processed, but available from January
1997 until ADEOS failure for same coverage as AVHRR data.
All airborne and satellite data avaialble for the project have been coregostered
and atmospherically corrected. In the case of the airborne data, the available estimates
of visibility were used along with the aircraft navigation data in conjunction with the
XX algorithm to correct for the presence of the atmosphere between the sensor and
the surface.