Gylbert Algorithm

The Algorithm gives a location within a 1°x1° spherical quadrilateral, relative to the quadrilateral's borders. The user picks whichever 1°x1° spherical quadrilateral which they are in.

Some cities, such as Calgary, Alberta, are cut into two or four parts by graticule boundaries. On the xkcd Blag, Gylbert suggested[1] that each week, Calgarians pick the location which is closest to exactly 51°N, 114°W, a spot in the center of the city. This effectively uses a graticule centered on 51°N, 114°W.

The Gylbert Algorithm is not equivalent to running the Algorithm hash on the shifted graticule, which would get you a fifth location different from all four surrounding locations. No-one is likely to be there.

The Gylbert Algorithm may be used at the discretion of individuals, because it does not change the target locations, merely picks one of them.