1 Spatial Modelling of the Terrestrial Environment - Georeferencial

184 Spatial Modelling of the Terrestrial Environment
Figure 9.3 Study of a wildfire spectral emittance analysed with ASTER imagery. The fire
was imaged during the night-time ASTER pass on 8 September 2000 over Zambia (16.09 ◦ S,
22.84 ◦ E). The image subset shows the 100 × 75 pixel ASTER 2.4 µm sub-image of the fire,
with a ground pixel resolution of 30 m. Bright pixels correspond to areas where fire activity
is causing intense thermal radiation at 2.4 µm wavelength. Similar emittance occurs in the
other five ASTER SWIR bands (1.6–2.3 µm) and spectra in all six ASTER bands for two of
the ‘fire’ pixels are shown in the graph, alongside the best-fire modelled spectra calculated
via the two thermal component model used previously to analyse spectra from ground-based
fires recorded with the GER 3700 instrument (see Figure 9.2). Spectra have been adjusted for
atmospheric effects using MODTRAN-estimated atmospheric transmissions convolved to the
ASTER bandpasses. The fit between the measured and modelled ASTER data of fire pixel 1 is
reasonable, but that for fire pixel 2 is poor because the individual spectral radiances do not
approximate a smoothly varying Planck-type function
However, applying this spectral-matching technique to satellite imagery poses the problem
that, unlike field spectroradiometers, the majority of satellite-based IR instruments
measure radiation in only a relatively few wavebands. This generally precludes use
of this method to derive the effective emitter characteristics from the data shown
in Figure 9.1. However, the Advanced Spaceborne Thermal Emission and Reflection
Radiometer (ASTER), orbiting onboard the EOS Terra satellite, does possess sufficient
shortwave IR wavebands for the application of this approach. A night-time southern Africa
ASTER scene (Figure 9.3, inset) was used to test the possibility of determining fire thermal
characteristics via this approach. Unfortunately, as can be seen from Figure 9.3, the individual
spectral measurements from the majority of the ASTER fire pixels were not well
fitted by a smoothly varying multi-thermal component Planck function of the type used
to analyse the emission spectra recorded by the GER 3700. In some cases a reasonable
fit was obtained, for example, in the case of Fire Pixel 1 in Figure 9.3, but it was found