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Methodology 52

The tasseled cap transformation was originally developed for understanding important

phenomena of crop development in the spectral space (Kauth and Thomas 1976).

However, with the information from the third dimension, i.e. the wetness feature, the

distinction between forest vegetation and cultivated vegetation is enhanced. Figure 4-5

adapted from Crist et al. (1986) illustrates the approximately locations of the scene

classes in the TM tasseled cap feature space. It gives the primary ideas about the types of

land cover enclosed in the test areas when the data distributions are presented in the

feature space. In the figure, the forest class is always distributed on the front of the “cap”

while the water class is located in the corner of the “cap”. The tasseled cap

transformation thus has potential in revealing key forest attributes including species, age

and structure (Cohen et al. 1995) and in extracting water and wetland pixels (Civco and

Hurd, 1999).

4.4.2 At-satellite reflectance-based tasseled cap transformation

Since the brightness feature highlights the areas of high reflectance, the greenness feature

the areas that are vegetated, and the wetness feature the areas that have high canopy and

soil moisture content, the wetland pixels can be extracted by using a tasseled cap

transformed imagery. An at-satellite reflectance-based tasseled cap transformation

compresses the Landsat 7 ETM+ multispectral data into a few bands associated with the

physical scene characteristics. Huang et al. (2001) developed a new tasseled cap

transformation based on at-satellite reflectance. They noted that their transformation was

more appropriate for regional applications where atmospheric correction was not feasible.

It also improves the ability to differentiate bright soil pixels from some dark green

vegetation pixels. The tasseled cap features can be derived through linear combinations

of the at-satellite reflectance coefficients as given in Table 4-2. Brightness is a partial

sum of all bands; greenness describes the contrast between the near infrared bands and

the visible bands; wetness depicts the contrast between the middle infrared bands that is

sensitive to water and other bands.

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