Research Methodology - Historical Land Use/Land Cover Classification Using Remote Sensing - page 74

Photographing
(recording)
parameters, senso
r calibration data

Output image
(corrected
reflectance
image)

Input image
(Gray-value
image)

Atmospheric
correction

Radiance image

Type of
atmosphere based
on the collected
weather data

Fig. 5.7 The general concept of atmospheric correction using ATCOR-2

Figure 5.7 illustrates the major followed steps in atmospheric correction of the
data set in this study. The solar zenith/sun elevation (61.07 8) and the solar azi-
muth/sun angle (126.22 8) were calculated using the sun position calculators based
on the recoding date of the image (e.g., p172r035)/(07.8.2007), scene-center-scan-
time (07:50:59 clock), and the Longitude (039 45 10 E) and Latitude (35 10 05 N)
of the scene center. The necessary information can be found in the header file of
the image data. The used atmospheric type was midlat-summer-rural, where:
midlat = radiation region of the mid-latitudes, summer = season, and
rural = aerosol type. Table 5.2 provides the used weather information. If the
meteorological data are not always obtainable, then the standard atmosphere (dry
desert) have to be used, which took into account the atmospheric effects in a good
approximation (Richter, ATCOR-2/3 User Guide, 2011).
The sensor calibration file represents another important input. This file includes
the calibration data (correction factors: Bias [c0] and Gain [c1]) of each channel.
Bias: Describes the spectral radiation on the sensor for a gray value of zero. Gain:
Represents the gradient calibration. The data takes place in the unit of electro-
magnetic radiation [mW cm -2 sr -1 lm -1 ] (Lillesand et al. 2008 ). ATCOR-2
calculates the reflection on the sensor using these factors in the linear equation:

Table 5.2 Weather data
from the Arraqah climatic
station (Longitude: 039 59 00
E; Latitude: 35 54 00 N;
Elevation: 250 m),
07.08.2007, 8.20 clock

Temperature 30.4 C
Relative humidity 44 %
Visibility 35 km
Air pressure 870.3 hPa
Sun elevation 61.07
Source The General Authority for Meteorology, Damascus, 2008