The term "remote sensing" refers to the techniques of measurement and interpretation of phenomena from a distance. Prior to the mid-1960"s the interpretation of film image was the primary means for remote sensing of the Earth"s geologic features. With the development of the opt mechanical scanner, scientists began to construct digital multi-spectral images using data beyond the sensitivity range of visible light photography. (46) These lineages are constructed by mechanically aligning pictorial representations of such phenomena as the reflection of light waves outside the visible spectrum, the refraction of radio waves, and the daily changes in temperature in areas on the Earth"s surface. Digital multi-spectral imaging has now be dome, the basic tool in geologic remote sensing from satellites. (47) The advantage of digital over photographic imaging is evident: the resulting numerical data are precisely known and digital data are not subject to the vagaries of difficult-to-control chemical processing. With digital processing, it is possible to combine a large number of spectral images. The acquisition of the first multi-spectral digital data sent from the multi-spectral scanner (MSS) aboard the satellite Land sat in 1972 consequently attracted the attention of the entire geologic community. Land sat MSS data are now being applied to a variety of geologic problems that are difficult to solve by-conventional methods alone. These include specific problems in mineral and energy resource exploration and the charting of glaciers and shallow seas. A more fundamental application of remote sensing is to augment conventional methods for geologic mapping of large are as. Regional maps present compositional, structural, and chronological information for reconstructing geologic evolution. (48) Such reconstructions have important practical applications because the conditions under which rock units and other structural features are formed influence the currency of ore and petroleum deposits and affect the thickness and integrity of the geologic media in which the deposits are found. Geologic maps incorporate a large, varied body of specific field and laboratory measurements, but the maps must be interpretative because field measurements are always limited by rock exposure, accessibility and labor resources. (49) With remote-sensing techniques it is possible to obtain much geologic information more efficiently than it can be obtained on the ground. These techniques also facilitate overall interpretation. Since detailed geologic mapping is generally conducted in small areas, the continuity of regional features that have intermittent and variable expressions is often not recognized, but in the comprehensive views of Land sat images these continuities are apparent. However some critical information cannot be obtained through remote sensing, and several characteristics of the Land 3at MSS impose limitations on the acquisition of diagnostic data. (50) Some of these limitations can be overcome by designing satellite systems specifically for geologic purposes; but to be most effective, remote-sensing data must still be combined with data from field surveys and laboratory tests, the techniques of the earlier twentieth century.