# Guyou hemisphere-in-a-square projection

Guyou doubly periodic projection of the world.

The Guyou hemisphere-in-a-square projection is a conformal map projection for the hemisphere (except for the four angles of the square where the conformality fails). It is an oblique aspect of the Peirce quincuncial projection. When it is used to represent the entire sphere it is known as the Guyou doubly periodic projection (see also doubly periodic function).

## History

It was developed by Émile Guyou of France in 1887 (Snyder, 1989).

## Formal description

It can be computed as an oblique aspect of the Peirce quincuncial projection by rotating the axis 45 degrees. It can also be computed by rotating the coordinates −45 degrees before computing the stereographic projection; this projection is then remapped into a square whose coordinates are then rotated 45 degrees (Lee, 1976). When both hemispheres are mapped with it, and placed next to each other, it is called Guyou doubly periodic.

## Properties

Its properties are very similar to those of the Peirce quincuncial:

• Each hemisphere is represented as a square, the sphere as a rectangle of aspect ratio 2:1.
• The part where the exaggeration of scale amounts to double that at the centre of each square is only 9% of the area of the sphere, against 13% for the Mercator and 50% for the stereographic (Peirce, 1879)
• The curvature of lines representing great circles is, in every case, very slight, over the greater part of their length. (Peirce, 1879)
• It is conformal everywhere except at the corners of the square that corresponds to each hemisphere, where two meridians change direction abruptly twice each; the Equator is represented by a horizontal line.
• It can be tessellated in all directions.