Nicolas Auguste Tissot

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Nicolas Auguste Tissot (French: [tiso]; March 16, 1824 – July 14, 1907) was a French cartographer, who in 1859 and 1881 published an analysis of the distortion that occurs on map projections. He devised Tissot's indicatrix, or distortion circle, which when plotted on a map will appear as an ellipse whose elongation depends on the amount of distortion by the map at that point. The angle and extent of the elongation represents the amount of angular distortion of the map. The size of the ellipse indicates the amount that the area is distorted.

Tissot's indicatrices viewed on a sphere: all are identical circles
The Behrmann projection with Tissot's indicatrices
The indicatrices demonstrate the difference between the 3D world as seen from space and 2D projections of its surface