Tissot's indicatrix

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In cartography, a Tissot's indicatrix (English: , French: [tiso]; plural: "Tissot's indicatrices"), or Tissot indicatrix, Tissot's ellipse, Tissot ellipse, or ellipse of distortion, is a mathematical contrivance presented by French mathematician Nicolas Auguste Tissot in 1859 and 1871 to characterize local distortions due to map projection. It is the geometry that results from projecting a circle of infinitesimal radius from a curved geometric model, such as a globe, onto a map. Tissot proved that the resulting diagram is an ellipse whose axes indicate the two principal directions along which scale is maximal and minimal at that point on the map. A single indicatrix describes the distortion at a single point. Because distortion varies across a map, generally Tissot's indicatrices are placed across a map to illustrate the spatial change in distortion. A common scheme places them at each intersection of displayed meridians and parallels. These schematics are important in the study of map...

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