The maps discussed up to this point have been general purpose maps or aerial photographs representing the largely physical character of the earth's surface. As we have noted already, in addition to representing the topography, the Canadian N.T.S. maps also show a considerable amount of information about the physical environment in general and about human activity as well. For example, lakes are shown as enclosed blue areas, forests by a green pattern of tree symbols, rivers by blue lines, and houses, churches, post offices, roads, bridges, water towers, quarries, and so on, are depicted by a variety of other special syols. We must now turn our attention to maps which depict just a single theme. These thematic maps, as they are called, use a variety of cartographic symbolization to depict the spatial pattern of a particular geographic quality or quantity rather than a variety of information of different sorts. The purpose of a thematic map is to give exclusive emphasis to a particular subject; its success is measured by the clarity, directness, accuracy and impact of the particular pattern being communicated to the map reader. As you might suspect, most maps used in the study of spatial information are thematic maps. Because they vary so widely in type and application, it will be useful to organize our discussion of them by the kind of data they show (qualitative or quantitative) and by the kinds of cartographic devices they employ.