Agriculture Reference
In-Depth Information
any spectrometer to a selected master spectrometer. They demonstrated that chemometric
analytical results are more coherent and precise. A global soil spectral library, not just as
with minerals, has to be followed by chemical information. Recently, Viscarra Rossel
(Rossel, Soil Spectroscopy Group 2009) initiated a spectral archive containing more than
10,000 spectra taken worldwide and is in the process of integrating them into this library
with some major soil attributes measured in the alternative “wet chemistry” way.
8.2 Field
In the field, there are more uncertainties than in the laboratory, such as atmosphere
attenuation, sun angle, aspect and slope of the sample area, large pixel sample, BRDF
effects and most of all, soil crusting and sealing which can emerge on any soil surface. The
latter prevent representation of the real soil body in the measurement. In the laboratory,
the soil is crushed to pass a 2 mm sieves, and all stones and litter debris are consequently
removed before the measurement. In the field, those materials are present, as are physical
and biogenic crusts and possible dust contamination. Another problem that might emerge
in field soil measurement is the adjacency effect and the mixed pixel. The first can occur
when the pixel in question is surrounded by bright reflective targets. The mixed pixel
problem occurs when a pixel is composed of several chromophores. It is simple when the
mixture is binary and complex when it is not. The sun's angle, slope and aspect might not
be factors in the field if artificial illumination is used in a standard procedure. A contact
probe equipped with tungsten-halogen illumination can be used, with caution. This is
because the soil surface measured by such an instrument may consist of a very narrow
field of view which might not represent the soil in question, but rather debris, stones or
even soil aggregation.
8.3 Airborne and spaceborne
Soil reflectance can also be measured from aircrafts and satellites, using either a point
spectroscopy (Karnieli et al. 2001) or IS (Ben-Dor et al. 2009) sensor. However, more
difficulties arise during such measurements to extract the correct reflectance, such as: small
integration time (less photons), strong atmosphere effects, large pixel size and varying
quality of the sensor's stability and sensitivity. Brook & Ben-Dor (2011) have recently
developed a more moderate method (SVC—Supervised Vicarious Calibration) to
standardize all sensors' radiometric readings, with the aim of deriving the optimal soil
reflectance from the airborne IS sensor. This method uses artificial net targets with varying
densities placed on a bright background area. It is easy to use and performs well. The
method has been recently validated in a European campaign over southern France, using
three different sensors simultaneously onboard two different airplanes. The preliminary
results were relatively good as compared to the alternative ways of deriving the reflectance
from the airborne sensor (unpublished data). The artificial target and the suggested method
help assess the atmospheric attenuation, and minimize sensor instability while correcting for
the systematic noise. Another limitation using airborne IS for soil is its high cost, low
availability, and the complexity involved in processing the raw data into a final reflectance
product (Ben-Dor et al. 2009). These factors actually prevent ordinary users from using this
technology, classifying it as an exclusive method. Vegetation coverage (partial or total) is
also a problem for deriving correct soil reflectance from afar when it is mixed in the sensor's
field of view.
