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The concept of scale is fundamental in the field of geographic information, but the term has several sometimes contradictory meanings. We distinguish between the cartographic scale, which is a ratio of relative size, and the level of detail or spatial resolution of the information, which is an absolute measure of the size of the entities referenced.

Cartographic scale is the ratio between the measured distance on the map and the distance measured in reality. Thus, on a large-scale map the relationship between these two distances is large, eg. 1: 5000, while a small-scale map is characterized by a smaller ratio, eg. 1: 500 000 (because 1/500 000 < 1:000). A large-scale map therefore covers a more limited scope than small-scale map. There is an inverse relationship between the scale and the detail of a map. A large-scale map usually shows more details than a small-scale map.

In everyday language, the word scale is used in the opposite sense. When we say that a phenomenon or a project is on a large scale, we mean that it covers a wide or large area, while a small-scale project concerns a very limited area. Paradoxically, maps use a small cartographic scale for visualizing a large-scale project while a large-scale map suffices for small-scale phenomenon that by definition covers a small area. In the same vein, work on a small scale will be characterized by a finer level of detail in objects and data manipulated and a higher accuracy in representations, while a large-scale work will be more general, less detailed and more broadly defined. It is advisable to always specify cartographic scale when using the term scale as in the first sense of ratio.

When we want to talk about the level of definition of information, ie. the size of the vector features or raster grid cells, we will discuss in the case of vectors the level of detail and in the case of raster mode, the resolution. Do not forget that a high level of detail or a high resolution means that raster cells or vector features are small. We sometimes think, wrongly, that the notion of scale has less importance now with digital data than it did during the era of mapping on paper. It is indeed easier to vary the scales of visualization by the effects of zooming in or out, but the constraints of the scale have not disappeared through the representation of the data on a screen rather than on paper. When we view a wide area and therefore a small map scale, very detailed data (thus captured from large-scale cartographic support) would make the map unreadable because the objects are too numerous and the fine detail would obscure the structures. Therefore, we generalize objects, i.e. simplify their forms, to make them meaningful in the chosen scale of representation.

See: Cartography, Geographic data, GIS Project, graphical and numerical scale

english/glossary/scale.txt · Dernière modification : //20/04/2015 16:09// de taien

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