Traditionally the interpretation of black-and-white aerial photographs has played a significant role in highway engineering. Recent increases in the availability of different forms of remotely sensed data and improvements in interpretative techniques have now resulted in recognition of remote sensing as a valuable tool for the highway engineer. This report summarises some of the new forms of image data and specialised interpretative techniques now available which can be selectively employed to provide more information than that previously gained from standard photointerpretation procedures. Proper use of these techniques requires a basic understanding of how the imagery is obtained, what it represents and its limitations with respect to spectral, spatial and brightness resolution, so that the best remote sensor data may be selected, processed and analysed for a particular problem to obtain the maximum amount of information with the least expense. Various forms of image enhancement or computer studies may be employed to assist image discriminations and classifications. Some enhancement techniques involve visual image analysis such as colour additive/subtractive viewing, stereoscopic and pseudo-stereoscopic photointerpretation. A few procedures are ordinarily accomplished through computer analysis (brightness ratioing, atmospheric correction etc), but others are effective with either imagery or numerical data. This latter group includes density slicing, contrast stretching, cluster analysis, pattern recognition, frequency analysis, and edge enhancement. Most procedures can be accomplished in several ways, with the accuracy of the results and efficiency of the operation largely dependent on the equipment used. Consequently, the economics of a project may often be the final consideration in the implementation of most interpretative techniques. (Author/publisher)
Abstract