Since its launch in July 1972, ERTS has generated multispectral data and images of a sizable portion of the world. While much analysis has been done by photointerpretation of the imagery, computer-implemented analysis of the digital tape data is increasing. At the same time, because of the large area coverage and synoptic view of the ERTS system, investigators are processing data from larger areas--in some cases up to several ERTS frames. The combination of detailed computer analysis and large area coverage has focused investigators on all factors which cause the data to vary--sensor system stability, atmospheric and solar illumination effects, and variations. in the reflectance of scenes, e.g., because .of phenological differences. To successfully conduct large area surveys which will demonstrate the ultimate utility of the ERTS system, all these factors need to be accounted for.