Projections fall into three main classes: Cylindrical projections are obtained by projection from the Earth's axis onto a cylinder touching the equator. Mercator's projection from the center of the Earth is a well-known example: its graticule (net of parallels and meridians) takes the form of a rectangular grid with the scale increasing toward the poles, which are infinitely distant; straight lines represent rhumb lines. Conic projections, best suited to middle latitudes, are obtained by projection onto a cone that caps the Earth, touching a given parallel. Azimuthal projections are from a single point onto a plane. The gnomonic projection, having the point at the center of the Earth, represents great circle routes by straight lines. The orthographic projection has the point at infinity, the projective rays being parallel; distortion is great, but the map looks like the globe. These geometric projections are now seldom used as such, but are modified to give correct relative areas, distances, or shapes. The scale of a map (assuming it to be constant) is the ratio of a distance on the map to the distance that it represents on the Earth's surface. It may be expressed directly as the ratio or representative fraction (1:63360), as a unit ratio (1in to 1mi), or by a graphic graduated scale. Maps use standard symbols and colors to show figures, giving the maximum information clearly. Types of maps include physical, political, economic, demographic, historical, geological, and meteorological maps; there are also star maps.
Maps have been drawn from earliest times, but until the Middle Ages most were little more than sketch maps based on impressions and guesswork except for those of the Greek geographers, notably Ptolemy of Alexandria. In the 14th century, Mediterranean sea charts were in use which were remarkably accurate, owing to the introduction of the compass and good estimates of distances sailed. The great voyages of discovery, the rediscovery of Ptolemy's map, and accurate surveying in the Low Countries revolutionized cartography in the 16th and 17th centuries, the work of Gerardus Mercator and Abraham Ortelius (who produced the first modern atlas) being well-known. Louis XIV promoted a national survey of France, the British Ordnance (1791) followed suit, and in the 19th century most civilized countries produced extensive maps. The US Geological Survey began in 1879. The International Map of the World (IMW), comprising about 1000 sheets at a scale of 1:1000000, was started in 1913 but is yet to be completed.
Related category GEOMETRY
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