a Landsat imagery pathrow 122065 b Ciliwung Watershed
Figure 4.2. Subset of Remotely Sensed Data to Focus Study Area.
4.1.3 Topographic Correction
Topographic correction refers to the compensation of the different solar illuminations due to the irregular shape of the terrain. This effect causes a
high variation in the reflectance response for similar land cover types: shaded areas show less than expected reflectance, whereas in sunny areas the effect is the
opposite. Therefore, the process of topographic normalization may be critical in areas of rough terrain, as a preliminary step to the multi-spectral and for multi-
temporal digital classification of landuse types.
Ciliwung has diverse topography type starting from 0 slope to 55 slope. In Landsat-7 ETM+ image, the area where the slope is over 15 such as
Cisarua and Puncak will have many shadows around it, especially forest area. In order to avoid misinterpretation during acquiring training area, then a process
called topographic correction should be performed first. In classification, process in acquiring training area is very crucial part. Therefore, acquiring training area
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In this study, topographic correction has been done by using Minnaert formula Equation 19. Model can be shown below:
should represent the real spectral value for each desirable class. Topographic correction process in this research used Minnaert method where the calculation
process done by using a model. Before the calculation process, all required data should be ready, which are DEM data, solar elevation angle value and azimuth
angle value from Landsat-7 ETM+ metadata acquisition date December 22, 2001. The picture of DEM used in this process can be seen in this Figure 4.3:
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Low 81 m High 3000 m
Figure 4.3. Digital Elevation Model DEM
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Entering data
Image corrected
Minnaert constant
Figure 4.4. Model of Topographic Correction
The value of solar azimuth angle for image 2001 is 122.4 while value of solar elevation angle is 56.6, Minnaert Constants can be seen in Table 4.3 as follow:
Table 4.3. Minnaert Constant k per Band
Band Minnaert Constant
1 -0.2644394098037027 2 -0.2371046780587166
3 -0.2144840868697183 4 -0.2724998585326885
5 -0.2780220865837141 7 -0.2099449197308662
The value of the Minnaert constant’s lies between 0 and 1. It is use to describe the roughness of the surface. Then Minnaert constant’s entered into the model and all
of digital number DN per band which found on image will be corrected so that
obtained an image which in the topography has been corrected. Figure 4.5 shows
the histogram performance of images as follow:
Figure 4.5. Histogram before Topographic Correction
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Figure 4.6. Histogram after Topographic Correction Histogram values indicate that there are a few value changes after topographic
correction but this are not to obvious visually. The comparison was also done
visually, that is no significant different, as shown in Figure 4.7:
After Before
Figure 4.7. Landsat 7 ETM+ 2001 before and after Corrected From the figure above, there is no change in image before and after topographic
correction process visually. But statistically, there is a change in spectral value
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before and after topographic correction was performed. In general, topographic correction is very assistive to reduce interpretation error of image classification.
4.2. Digital Image Processing
