FALSE TOPOGRAPHY PERCEPTION PHENOMENA AND ITS CORRECTION

The remote sensing data of a rugged terrain always suffer from topogra phic effects and, in such cases people frequently perceive valleys as ridges, and vice versa in the optical imagery of Sun-synchronous satel lite. The appearance of inverse topography can be termed as false topo graphy perception (FTP). FTP has been observed in various satellites i mages such as Landsat, IRS, and SPOT. The problem of FTP becomes more severe in high altitude areas of hilly terrain, such as Himalayan terr ain. FTP is usually caused by the combination of various interrelated factors; among them are topographic relief, Sun elevation the azimuth angle, viewing angle, and hatching or engraving features present on th e slopes. Once satellite data of rugged terrain are acquired, then all FTP-influencing factors are fixed, except for the viewing angle of th e observer. Hence, the correction of FTP becomes straightforward. Ther e are two ways to correct satellite data for FTP effects: (a) rotate t he image by between 135 degrees-180 degrees, i.e., changing the viewin g angle of the observer; or (b) inverse the image data by making a neg ative of the whole image (255-image), which will bring change in the S un's illumination by 180 degrees. The present paper discusses the FTP phenomena and its causes, and suggests methods of correction.