The Significance and Sustainability of Charcoal Production in the ...

the electromagnetic radiation (EMR) from those objects. Spectral radiance is defined as the
energy at a specific wavelength radiated by an object of unit area per solid angle of
measurement (watts per square metre per steradian per micrometer: W m-2 sr-1 �m). The
spectral composition of the signal is modified by interactions with the atmosphere both on the
upward and on the downward journey. Because atmosphere is not constant its composition
either in space or time, the level of EMR recorded by the sensor is not the same. The removal
of the effects caused by atmospheric interaction remains one of the fundamental problems of
RS (Mather 1992: 60-61). Pre-processing is done in order to remove, as far as possible, the
‘external’ effects in the image; sensor calibration, correction for atmospheric, illumination and
viewing geometry effects, and to conduct geo-referencing which refers to the registration of
the image to an accepted map projection (ibid. 60).
According to the preprocessing level 2A of acquired SPOT data, the geometric correction has
been conducted, i.e. the data has been orthorectified to a UTM projection with a WGS-84
spheroid and datum. Survey of Kenya digitized topographic map mosaic that contains grid
coordinates was used as reference image for an ‘image-to-map’ rectification that was
conducted for the 2005 SPOT 4 image in ERDAS IMAGINE setting the Geometric Model to
polynomial and using the geo-correction tools to choose ten ground control points (GCPs)
from the reference map before automatic transformation calculation that gave the RMS error
figure of 0.3409 which is less than 1 (pixel) as recommended. A linear transformation (an
equation of polynomial order 1) was used because the original and desired output projection
systems are actually very similar. The image was saved, or re-sampled, with nearest
neighbour option. An ‘image-to-image’ rectification was done thereafter using the previously
rectified image as reference image and the 2006 SPOT 4 image as input image. The RMS
error figure was 0.3852. The same procedures were conducted for the 2011 images, the 2011a
image that was rectified using 2005 image as reference gave RMS error 0.4921 and the 2011b
image that was rectified using 2006 image as reference 0.4905.
As the context area is gently undulating lowlands there was no need for topographical
correction. Neither atmospheric correction was conducted as it was not considered necessary.
Radiometric correction is done to normalize the sensor. The header file of the SPOT offer the
gain and offset values that are needed in converting count values to radiance or reflectance.
The sensor gain and offset vary over the life of the sensor which leads to that a raw pixel
value (DN) cannot have any absolute meaning (Mather 1992: 60). DN values in the used
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