Image Quality and Principles of Interpretation
The amount of information that can be extracted from a remote sensing data product (photograph or image) is primarily dependent upon its quality. It is therefore necessary first to have an idea of all the factors which affect image quality, before proceed
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7.1 Image Quality The amount of information that can be extracted from a remote sensing data product (photograph or image) is primarily dependent upon its quality. It is therefore necessary first to have an idea of all the factors which affect image quality, before proceeding to interpretation and applications. Basically, there are two aspects to image quality radiometric and geometric. Both collectively govern the amount of extractable information. The geometric aspects of images and photographs have been discussed in Cha pter6. Here, we shall focusoura ttention on the radiometric aspect of image quality. During visual interpretation, objects on an image or photograph are discerned from one another primarily by relative differences in brightness or colour. It is a common experience that a bright object can be easily marked if located on a dark background. However, the same object may be difficult to locate against a bright background (Fig. 7.la,b). Object or Brightness Contrast Ratio, i.e. the ratio of brightness of any two objects, is an important factor in deciding to what degree any two features occurring side by side on an image or photograph can be differentiated by visual inspection. Sometimes the term contrast ratio is used to denote the ratio between the maximum and minimum brightness in anyone scene. For this, however, we prefer to use the term dynamiC range. In general, a remote sensing product displaying good dynamic range is said to be of good image quality and an image with low dynamic range is termed fiat or washed out. Digital processing techniques are available to enhance image contrast ratio and dynamic range in images (see Sect. 12.4.1). Here, we shall discuss some of the more basic factors which govern the radiometric quality of images. Radiometric quality of photographs and images depends upon three groups of factors: (I)
R. P. Gupta, Remote Sensing Geology © Springer-Verlag Berlin Heidelberg 1991
ground/terrain properties, (2) environmental factors during data acquisition and (3) sensor instrumentation factors (Sabins 1987; Moik 1980) (Table 7.1).
7.1.1 Ground Properties Lateral variation in the ground properties (namely spectral reflectivity, thermal properties like thermal inertia etc., as the case may be) across the scene influences the general radiometric image quality. Detecting such variations in inherent ground properties across the scene is the crux of the problem and fashions the scope of remote sensing applications. The ground properties affecting response in the optical region, and potentialities of remote sensing techniques in discerning such variations/parameters are discussed elsewhere (Chaps. 8,9,13).
7.1.2 Environmental Factors 1. Solar illumination and time of survey. Energy conditions on the ground depend on the azimuth and angle of sun elevation and time of survey. Remote sensing data should be acquired at a time when energy conditions are optimum for detecting differences in ground properties, and are quite stable. In solar reflection region of the spectrum. noon-time s
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