Measuring distortion using Tissot's Indicatrix

In the nineteenth century, Nicolas Auguste Tissot developed a method to analyze map projection distortion. An infinitely small circle on the earth's surface will be projected as an infinitely small ellipse on any given map projection. The resulting ellipse of distortion, or indicatrix, shows the amount and type of distortion at the location of the ellipse. For example, if an indicatrix is elongated from north to south, shape is correspondingly distorted at that location on the map. The same goes for east–west stretching or oblique stretching. On a conformal map, the indicatrices are all circles, but they vary in size. On an equal area projection, the indicatrices have varying ellipticity, but the same area.