The map projection becomes critical in understanding how scale varies throughout the map. In maps covering larger areas, or the whole Earth, the map's scale may be less useful or even useless in measuring distances. If the region of the map is small enough to ignore Earth's curvature, such as in a town plan, then a single value can be used as the scale without causing measurement errors. In this case 'scale' means the scale factor (also called point scale or particular scale). It is the ratio of the mapped point's scale to the nominal scale. The second distinct concept of scale applies to the variation in scale across a map. Many maps state the nominal scale and may even display a bar scale (sometimes merely called a "scale") to represent it. The ratio of the Earth's size to the generating globe's size is called the nominal scale (also called principal scale or representative fraction). The generating globe is a conceptual model to which the Earth is shrunk and from which the map is projected. The first way is the ratio of the size of the generating globe to the size of the Earth. Because of this variation, the concept of scale becomes meaningful in two distinct ways. This simple concept is complicated by the curvature of the Earth's surface, which forces scale to vary across a map. The scale of a map is the ratio of a distance on the map to the corresponding distance on the ground. A map would also usually give its scale numerically ("1:50,000", for instance, means that one cm on the map represents 50,000cm of real space, which is 500 meters) A bar scale with the nominal scale expressed as "1:600 000", meaning 1 cm on the map corresponds to 600,000 cm=6 km on the ground. For its graphical representation, see Linear scale. This article is about the general mapping concept.
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