Agriculture Reference
In-Depth Information
As a further step, the retrieval of quantitative mapping using spectral parameters derived
from hyperspectral images should take into account differences in the spectral and spatial
resolution between ground spectrometer and image data (Choe et al. 2008).
8.4 General issues
In all of the above domains, it must be remembered that reflectance spectroscopy is strongly
affected by water content, particle size distribution and the measurement protocol. In
addition, ways of calculating the reflectance from radiometric readings, i.e. against a white
standard panel or using the radiance-to-irradiance ratio, may change the final product.
Furthermore, the reflectance represents only the surface and cannot provide information on
the soil profile (unless a penetrating fiber optic is used such as that described by (Ben-Dor et
al. 2008). When obtaining a soil spectrum from a user, a meta-data set which characterizes
all of these factors (moisture content, grain size and method of reflectance calculation) is
strongly needed, because they have a significant impact on the final spectra's behavior.
8.5 Summary
Soil reflectance is an inherent property of the soil but many factors can affect its
performance. Internal standards, a standard protocol and controlled conditions are a few of
the things that can assist in sharing and comparing soil spectra (and chemometric models)
worldwide (e.g. the Global Soil Library by Rossel and Soil Spectroscopy Group (2009)). It is
obvious that uncertainties in the laboratory are smaller than those in the field, and the latter
are smaller than those obtained from air or space. Use of radiometric data acquired from
remote sensing domains to measure soil reflectance information should therefore be
undertaken with caution.
9. Future potential of remote sensing technology
for assessing soil
contamination
Soil reflectance has become a very useful tool over the past 20 years in the laboratory, in the
field and from air and space. As the sensitivity of portable field spectrometers increases,
field soil spectroscopy is expected to become a basic tool for rapid point-by-point
monitoring of the soil environment. The commercial development, operation and use of air-
and spaceborne image spectrometers can provide near-laboratory-quality spectra of every
pixel in an image and very soon, will permit remote sensing of soils with high standards.
Information about soils from reflectance spectra in the VNIR and SWIR spectral regions
represents almost all of the data passive solar sensors can provide. It is anticipated that the
thermal IR regions will also become part of the soil spectral arena as they contain diagnostic
information on some soil attributes that the reflective spectral region does not. The
development of a thermal spectrometer (either point or imaging) will enable obtaining more
spectral information with better accuracy. Another future insight is the capability to
measure the soil profile's spectral distribution using a fiber optic assembly and small
boreholes. Today, the major limitation of this technique is the fiber optics' length (as the
fiber length increases, the signal across the SWIR region decreases—today, the length is
limited to 1.5 m). Miniaturizing the spectrometer will enable placing it on the penetrating
optical head, thus bypassing the limitation imposed by fiber length. This, in turn, will enable
spectral measurement of any profile depth which, when combined with the NIRS model,
might be able to provide
in-situ
soil contamination attributes of the soil profile. Combining
