The image may be described in strictly numerical terms on a three-coordinate system with x and y locating each pixel and z giving the DN, which is displayed as a gray-scale intensity value. Digital numbers range from zero to some higher number on a gray scale. Each pixel also has a numerical value, called a digital number (DN), that records the intensity of electromagnetic energy measured for the ground resolution cell represented by that pixel. The position of any picture element, or pixel, is determined on an xy coordinate system in the case of Landsat images, the origin is at the upper left corner of the image. One can think of any image as consisting of tiny, equal areas, or picture elements, arranged in regular rows and columns. The processes are illustrated with Landsat examples because these are the most familiar and are readily available to readers the digital processes, however, are equally applicable to all forms of digital image data. The publications cited at the end of the chapter describe in detail the methods and the mathematical transformations. This chapter describes and illustrates the major categories of image processing. A third impetus is the continued development of faster and more powerful computers, peripheral equipment, and software that are suitable for image processing. Another impetus was the Landsat program, which began in 1972 and provided repeated worldwide coverage in digital format. The low quality of the images required the development of processing techniques to make the images useful. For remote sensing, the initial impetus was the program of unmanned planetary satellites in the 1960s that telemetered, or transmitted, images to ground receiving stations. Many image-processing techniques were developed in the medical field to process X-ray images and images from sophisticated body-scanning devices. Indeed, more books are available that deal with image processing than with image interpretation, which is ironic because processing is really just a preparatory step for the all-important activity of interpretation.ĭigital processing did not originate with remote sensing and is not restricted to these data. At the other extreme, large mainframe computers are employed for sophisticated interactive manipulation of the data to produce images in which specific information has been extracted and highlighted.Ī number of books have been published on the subject of digital image processing a representative selection is listed at the end of this chapter. The simplest form of digital image processing employs a microprocessor that converts the digital data tape into a film image with minimal corrections and calibrations. Many types of remote sensing images are routinely recorded in digital form and then processed by computers to produce images for interpreters to study. Remote sensing principles and interpretation. Reproduced, with permission, from:Sabins Jr., F.
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