The separation of maps into sets according to criteria uniquely characterising each set. Maps are commonly classified according to (a) what geometric quality of the mapped region is preserved unchanged in the map; (b) the ratio (scale) between distances in the mapped region and lengths of corresponding lines in the map; (c) the region represented by the map; (d) the intended use of the map; (e) the type of information shown; or (f) some measure of the accuracy of the map. (a) Maps are classified as equidistant, conformal, azimuthal, or equal area according as they keep unchanged, except for scale, distances from a given point or line, angles between lines, azimuths from a given point, or areas within closed curves. An additional category is that of maps which introduce the smallest possible error (distortion) into the map under specified conditions (minimal distortion maps). (b) Maps are classified as large scale, medium or intermediate-scale, or small scale maps according as the average ratio between distances on the map and the corresponding distances on the ellipsoid is larger than some number used as criterion for the set, lies between that number and some smaller number selected as criterion, or is smaller than that second number. The criterion for large scale maps is usually 1/75 000 or larger; that for medium scale maps is usually larger than 1/1 000 000 - 1/500 000 is a common number. (c) Maps are classified as world maps, national maps, State maps, etc. , according to the size and location of the region represented. A world map (also called a global map) represents the entire Earth, a map of North America represents the continent of North America, etc. (d) There is no limit to the variety of uses to which a map may be put or for which it may be intended. However, some categories based on intended use are quite large: aeronautical charts and nautical charts take many different forms and the set contains many subsets; military maps, for tactical or strategic planning; route maps, for travelling by various means; and so on. (e) Classification according to the type of information shown is almost equivalent to classification according to intended use. In fact, it differs from that classification principally in having a general purpose category called topographic maps which show, symbolically, just about everything one would see if viewing the ground from a point well above it. Maps which are not topographic maps, i.e., which show only selected details or give, graphically, non topographic information are called thematic maps. Typical are maps showing the geologic structure of the crust (geologic maps), density of population (population density maps), the use to which land is being put (land use map), or the ownership of land (cadastral maps). (f) Classification of maps according to their accuracy is fairly standard for topographic maps, although the scheme varies in detail from country to country and from application to application. In the United States of America, a map of standard accuracy has been defined as one (a) meeting the National Map Accuracy Standards, (b) representing all features by symbols and treatments approved at the time of publication, (c) portraying cultural detail with reasonable completeness at the time of appraisal, and (c) having a contour interval representing all significant hypsometric detail (considering the scale of the map. ) Each actual map is then classified according to the extent to which it meets these standards, as follows: class 1 maps meet all four criteria; class 2 maps meet three of them, class 3 maps meet two of them, class 4 maps meet one or none of them, and class 5 maps may meet any or all of these criteria but have been superseded by maps of better quality.