Geo Lounge’s Elizabeth Borneman has a piece on Tissot’s indicatrix, which tends to turn up in discussions of map projections. (See, for example, this piece from Vox’s Johnny Harris, and the accompanying video.) And for good reason: it’s a useful visualization tool. All maps distort—representing a curved surface on a flat plane, et cetera, et cetera—but a grid of Tissot’s indicatrices superimposed on a world map will measure the distortion—linear, angular, by area—produced by that map’s projection. On the Mercator projection, the indicatrices remain perfect circles, but grow larger toward the poles; on equal-area equatorial projections, they maintain their size but squish into ellipses; on other projections they also angle left or right depending on how close they are to the edge. On compromise projections like the Robinson (see above), they do all three.