Remote Sensing - a tool for environmental observation

Figure 47 example of a thermal image for intelligence

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Figure 4.7 Example of a thermal image for intelligence. Figure 4.8 Thermal image of a human body,
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62 Chapter 5 Image Corrections and Analysis Techniques 5.1 Introduction Many types of remotely sensed images (Landsat, SPOT, NOAA, ERS etc.) are recorded in digital form. A computer is needed to process these data. Moreover, a computer is very useful to process the large volume of data acquired by remote sensing systems. One Landsat TM scene, for example, consists of 7 spectral bands covering an area of 185 by 185 km with pixels of 30 by 30 m. The data volume in bytes can be computed as: - number of pixels in one image line 185 km/ 30 m = ± 6165 pixels; - number of image lines: 185 km/ 30 m = ± 6165 lines; - 7 spectral bands (including the thermal band); The total data volume is 6165*6165*7 = 266 Megabytes. Processing by hand is virtually not possible. Often Landsat images are processed in quarter scenes. Quarter scenes cover an area of 90 by 90 km and their data volume is approximately 67 Mbytes, which can mostly be processed easily on image processing systems. Image processing is defined as: ‘digital techniques, consisting of a number of operations and modules as expedients to elucidate the information contents of images to the utmost’ (an interactive approach of the data analysis and image interpretation is common practice). Hence, image processing comprises all manipulations of digital images such as geometric corrections, radiometric corrections, image enhancements, image filtering, data reduction techniques and image classifications. This chapter focuses mainly on processing of remote sensing images acquired in the optical wavelength such as Landsat TM, Landsat MSS and SPOT because environmentalists will mainly make use of these types of images. 5.2 Digital Image Structure A digital recorded image consists virtually of a number of layers (or matrices) of numbers or digital numbers. The intensity value of a pixel (reflectance or thermal emission) is recorded as digital number or DN. Each layer represents a spectral band and each layer consists of a number of columns and rows (i.e. image lines). Each number per layer is the Digital Number (the measured radiance) per picture element or pixel. The measured radiance is mostly stored as a byte (8 bits), values can vary from 0 to 255 and calibration data are needed to transfer them to reflectance or absolute radiance. Sometimes, digital recorded data are stored as 10 bits e.g. NOAA-AVHRR or 16 bits (words) e.g. AVIRIS. Landsat data are stored as a byte. A Landsat TM image consist of 7 layers or matrices (because it has 7 spectral channels) and it has approximately 6165 image lines with approximately 6165 pixels on each image line. Figure 5.1 and 5.2 illustrates the structure of an Landsat TM scene. The structure of a digital recorded image is sometimes called an ‘image cube’. The X- and Y- coordinates within the matrix of DN represent the position of each pixel in the field. The Z-
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63 coordinate represents the spectral position (1 to 7 for TM). As is already discussed, the spatial resolution of the thermal band of Landsat TM is 120 by 120 m. When it is stored in a digital
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