![]() ![]() Data sets are geographic information their collection depends on the chosen datum (model) of the Earth. Because maps have many different purposes, a diversity of projections have been created to suit those purposes.Īnother consideration in the configuration of a projection is its compatibility with data sets to be used on the map. The purpose of the map determines which projection should form the base for the map. ![]() Each projection preserves, compromises, or approximates basic metric properties in different ways. Similarly, an area-preserving projection can not be conformal, resulting in shapes and bearings distorted in most places of the map. Because the Earth's curved surface is not isometric to a plane, preservation of shapes inevitably requires a variable scale and, consequently, non-proportional presentation of areas. Map projections can be constructed to preserve some of these properties at the expense of others. ![]() Many properties can be measured on the Earth's surface independently of its geography: Metric properties of maps Īn Albers projection shows areas accurately, but distorts shapes. The National Geographic Society and most atlases favor map projections that compromise between area and angular distortion, such as the Robinson projection and the Winkel tripel projection. : 156–157 To contrast, equal-area projections such as the Sinusoidal projection and the Gall–Peters projection show the correct sizes of countries relative to each other, but distort angles. However, it has been criticized throughout the 20th century for enlarging regions further from the equator. : 45 This map projection has the property of being conformal. The most well-known map projection is the Mercator projection. Therefore, more generally, a map projection is any method of flattening a continuous curved surface onto a plane. The surfaces of planetary bodies can be mapped even if they are too irregular to be modeled well with a sphere or ellipsoid. The Earth and other large celestial bodies are generally better modeled as oblate spheroids, whereas small objects such as asteroids often have irregular shapes. Most of this article assumes that the surface to be mapped is that of a sphere. Few projections in practical use are perspective. Rather, any mathematical function that transforms coordinates from the curved surface distinctly and smoothly to the plane is a projection. However, the term "map projection" refers specifically to a cartographic projection.ĭespite the name's literal meaning, projection is not limited to perspective projections, such as those resulting from casting a shadow on a screen, or the rectilinear image produced by a pinhole camera on a flat film plate. More generally, projections are considered in several fields of pure mathematics, including differential geometry, projective geometry, and manifolds. There is no limit to the number of possible map projections. The study of map projections is primarily about the characterization of their distortions. Depending on the purpose of the map, some distortions are acceptable and others are not therefore, different map projections exist in order to preserve some properties of the sphere-like body at the expense of other properties. Projection is a necessary step in creating a two-dimensional map and is one of the essential elements of cartography.Īll projections of a sphere on a plane necessarily distort the surface in some way and to some extent. In a map projection, coordinates, often expressed as latitude and longitude, of locations from the surface of the globe are transformed to coordinates on a plane. In cartography, a map projection is any of a broad set of transformations employed to represent the curved two-dimensional surface of a globe on a plane. A medieval depiction of the Ecumene (1482, Johannes Schnitzer, engraver), constructed after the coordinates in Ptolemy's Geography and using his second map projection
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