1 Spatial Modelling of the Terrestrial Environment - Georeferencial

Fireline Intensity and Biomass Consumption in Wildland Fires 185
that the majority of the pixels more closely resemble the situation of Fire Pixel 2. One
explanation for this is provided by the ASTER technical specifications (ERSDAC, 2001),
which suggest the ASTER inter-band spatial registration is accurate to only 0.2 pixels.
When coupled with this imprecise inter-band registration, a fire signal that varies strongly
at the sub-pixel scale could easily induce the type of spectra shown in Fire Pixel 2. One
solution to this problem might be to analyse ASTER pixel groups rather than each pixel
individually, essentially by calculating the total spectral emittance from a fire via the sum of
the spectral radiance recorded in each fire pixel at each wavelength. The resultant composite
spectra could then be used to determine the effective emitter characteristics for the entire
fire. Unfortunately, this approach is also limited in its application, this time by the fact that
for any particular fire imaged by ASTER, a number of the pixels are saturated, particularly
in the longer wavelength SWIR bands, so that the total fire thermal emittance cannot be
accurately determined. We must therefore conclude that current spaceborne sensors appear
not to allow the multi-thermal component spectral fitting approach to be used to determine
FRE, although the hyperspectral data from the EO-1 Hyperion experimental sensor remain
to be investigated.
Fortunately, an alternative method for FRE derivation exists, whereby measurement
of the energy emitted by a fire in one particular, well-chosen, spectral region provides the
ability to estimate the amount of energy emitted over all wavelengths. This method was first
proposed by Kaufman et al. (1996) and was first tested with data of active fires obtained
by the MODIS Airborne Simulator and the Airborne Visible/IR Imaging Spectrometer
(AVIRIS). Later Kaufman et al. (1998a) developed empirically derived equations relating
the fire pixel brightness temperature recorded in the middle IR (MIR) 3.9-µm channel of the
MODIS spaceborne sensor, which also orbits on the EOS Terra (and Aqua) satellites, to the
total energy emitted by all flaming and smouldering activity within that pixel. Fortunately,
in comparison to the ambient temperature background, fires emit so strongly in the MIR
spectral region that pixels containing fires significantly smaller than one hectare are easily
identified in remotely sensed imagery of kilometre-scale spatial resolution, making FRE
derivation from a wide range of fires possible via this approach. Building on the work
of Kaufman et al. (1998a), an alternative method for the derivation of FRE from MIR
radiance data has been derived via a more physically based approach, which allows the
resultant equations to be easily adapted to sensors of differing spatial and spectral resolutions
(Wooster et al., 2003).
9.4 Derivation of Fire Radiative Energy from MIR Spectral Radiances
9.4.1 Theory and Testing with Polar-Orbiting Satellite Imagery
The physically based method of deriving FRE from measurements of the MIR radiance
(L) recorded at the fire pixel is based on the fact that, over any particular temperature
range of interest, the radiance emitted by a blackbody at a particular wavelength can be
approximated by a simple non-linear function (Wooster and Rothery, 1997):
L λ (T ) = aT b (2)
where λ is the wavelength (m), T is the temperature (K), L is the spectral radiance (W m − 2
sr −1 m −1 ) and a and b are empirically derived constants, dependent upon both wavelength
and temperature range used.