The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008
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This paper describes the dual-view FIGOS data acquisition and
storage, the generation of metadata and the pre-processing
applied prior to a BRDF retrieval in the context of the spectral
database SPECCHIO (Huni et al., 2008), which is used to store
the according spectral data and metadata.
Table 2: Illumination and reflection geometry configurations in
the field case (Schaepman-Strub et al., 2006)
The hemispherical conical case (Table 2) is the configuration
that describes the illumination/reflection geometry under field
conditions. Under clear sky conditions most energy is stemming
from the direct solar irradiation while a minor part is
contributed by the indirect radiation originating from scattering
processes in the atmosphere and from adjacent objects. The
indirect component is inhomogeneous over the incoming
hemisphere: largest values are observed close to the direct
irradiance direction and minimum values in the principal plane
opposite the sun position. For clarity, we will refer to the
hemispherical-conical configuration under field conditions as
“Field HCRF” within this paper.
2. DATA ACQUISITION AND PREPARATION
2.1 Data Acquisition
Acquisition of spectrodirectional datasets is based on the dual
view FIGOS. Two ASD FieldSpec 3 spectroradiometers
(Analytical Spectral Devices Inc., 2007) are mounted on a
sledge running along a zenith arc and capture the down-welling
(Linc diff) and up-welling (L r ) radiances (Schopfer et al., 2007a).
Due to saturation and pointing problems, the direct illumination
E dir is not measured with the upward looking ASD but with an
MFR sunphotometer (Yankee Environmental Systems Inc.,
2000) that collects the incoming total and diffuse radiance in six
narrow spectral bands (central wavelengths: 413, 496, 612, 671,
867, 935 nm) and one broadband channel.
The BRDF is the most fundamental of the configurations shown
in Table 1, as all others may be derived from it by integration
over the illumination and reflection angles. The provision of
accurate, spectral albedo imagery products (e.g. bi-
hemispherical reflectance), as is foreseen for the APEX
(Airborne Prism Experiment, (Nieke et al., 2005)) processing
facility, depends on knowledge of the BRDF of the sampled
objects.
It is thus the goal the retrieve the BRDF from empirical data
collected under the field conditions, i.e. from Field HCRF data.
An according retrieval algorithm has been proposed by
Martonchik (1994) and was experimentally applied to dual
view FIGOS (Field Goniometer System) data by Schopfer et al.
(2007a).
The BRF (Bidirectional Reflectance Factor) is the BRDF
normalized to the reflectance characteristics of an ideal
Lambertian reflector (BROr^t,^ = zt' 1 ) and is calculated by
dividing the BRDF by the factor of 7i. The BRF is retrieved
from field data by iteratively solving the following equation for
R (Schopfer et al., 2007a):
- *,)= a-' 1 • R(e, AA, - K\ EM)+
X l i ]r{8,AA-«*,)' L%(0„0„A, - A)- sin (*,)• cos^yw,
0 0
(Eq. 1)
Where
Q.,Q r = illumination and view zenith angles
<j). r - <j) {) = illumination or view azimuth angle relative
to the solar principal plane
R(0 r , 0 t , <j) - <f>^ = Bidirectional Reflectance Factor (BRF) of
the target for the given illumination angles.
Prerequisites of the retrieval are the quantities L r , E dir and
Lincjiff; which should be measured in the field. However, due to
the instrumentation used, field data must be subjected to pre
processing before the retrieval can be started.
Data collection for one target is usually carried out during one
day with varying illumination angles, resulting in several dual
view goniometer data sets.
2.2 Data Structuring
Organised data collection 'is an important issue and the
structuring of the data in a defined way not only helps the
acquisition process but also aids subsequent data handling.
The two instruments capturing the incoming and reflected
radiances are being controlled by two independent laptops.
Thus, these data must be combined into one dataset before
loading them into the database. Furthermore, dual-view
goniometer data sets must be divided into sky/target and
reference panel measurements. The latter are taken according to
a predefined sampling protocol and are of importance for (a)
conversion of radiances to reflectances and (b) calculations
needed during the pre-processing, as will be detailed later on.
A hierarchical structure thus holds the data (ASD and MFR
files) for one target and must be set-up and filled accordingly
prior to the loading into SPECCHIO (Figure 1).
▼ m 19.2.2008
► Si hem_a
► Si hem_b
► £i hem_c
► Si hem_d
► SB hem_e
► ¡jjhem_f
► il hem_g
▼ Ü hem_h
▼ Hi incoming
► Ü reference
► Qj sky
▼ in reflected
► £i reference
► S target
▼ m mfr7
► S3 total
► S diffuse