Forest Fire Monitoring with SPOT-4 Satellite Imagery
2.3 New Representation
One disadvantage of the 432 representation is that the smoke plumes tend to be too thin for the purpose of determining the size of the plumes. Thus, we propose a new way of displaying the SPOT-4 images. This is achieved by replacing the NIR band by the average of NIR and Green bands in the 432 representation. In this manner, both the active fires and smoke plumes are clearly visible. In figure 3, the new way of displaying scheme is applied to the same image displayed in figure 1. The smoke plume of the big fire on the top left corner is now clearly visible as compared to the right image shown in figure 1.
3. Estimation of Temperature
3.1 Radiance Received
The radiance received by the satellite can be written as
where L
p(
l ) is the path
radiance resulting from multiple scattering of
solar radiance by air molecules and aerosols,
L
r(
l
) is the solar radiance reflected from the ground and L
e is the thermal radiation emitted by earth surface. Both the reflected radiance L
r and emitted radiance L
e have suffered from atmospheric attenuation. However, in SWIR band, the atmospheric effect can be ignored as far as this paper is concerned. The emitted radiance can be modelled by Planck's radiation equation
where h is the Planck constant, c is the speed of light, k is the Boltzmann's constant and T is the absolute temperature. The reflected radiance depends on the ground reflectance and solar radiance falling onto it.
3.2 Radiometric Calibration of SPOT-4
According to the SPOT's interface document, the level 2A image of SPOT-4 can be calibrated to physical radiance by
L = X / AG
where X is the digital value of level 2A image, A is the absolute calibration coefficient and G is the programmable gain. Both A and G can be found in the leader file.
3.3 Effective Wavelengths
In order to use Planck's
equation to estimate the temperature, we need to
know what wavelength to use in the equation. The
effective wavelength,
l
e is the one which best fit the following equation
where f(
l
) is the spectral response of the SWIR band as shown in figure 4. The equations are computed with T range from 273 to 373 K. The effective wavelengths for band 4 of instruments HRVIR1 and HRVIR2 are found to be 1649 nm and 1635 nm respectively.
Figure 4 SWIR Spectral responses of the two HRVIR instruments
