Mapzh-cn:地图Map
A map is a two-dimensional representation of a three-dimensional space. The science of making maps is called cartography. Early maps were vague and there was often controversy as to where to centre the map - one world map, for instance, has Jerusalem at the centre. Many maps have a scale, determining how large objects on the map are in relation to their actual size. A larger scale shows more detail, thus requiring a larger map to show the same area. Some, though, are not drawn to scale - a famous exmmple being the London Underground map If the map covers a large area of the surface of a globe, such as the Earth, it also has a projection, a way of translating the three-dimensional real surface of the geoid to a two-dimensional picture. The most commonly used is the Mercator Projection; other popular projections are polar and a variety of equal-area projections. The features shown on a map vary according to its purpose. For example, a road map may or may not show railroads, and if it does, it may show them less clearly than highways. Maps can be political or geographical. The most important purpose of the political map is to show territorial borders; the purpose of the geographical is to show features of physical geography such as mountains, soil type or land use. Geological maps show not only the physical surface, but characteristics of the underlying rock, fault lines, and subsurface structures. Many mapping projects have been carried out by the military. An example of this the British Ordnance Survey (now a civilan government operation). Because maps are abstract representations of the world they are not neutral documents and must be carefully interpreted.
For maps on a computer display, e.g. from the web or locally stored on CD-ROM or harddisk, zooming in may mean one of the following:
For a single raster graphics image the second applies until the pixels in the image file correspond to the pixels of the display; on further zooming in, the third applies. A variation of the third possibility is that interpolation is performed. Text is not necessarily enlarged when zooming in. Similarly, a road represented by a double line may or may not become wider when one zooms in. A variation of the first possibility above is that more text is displayed (such as more town names), but that for the rest of the image the second applies.
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