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 four (or two) 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.

Related ideas

Two xkcd blag posts later, John suggested [2] that each user simply pick the nearest point, ignoring the graticules entirely. This is a generalization of the Gylbert Algorithm.

ZanderM, Tyler, and others suggested[3][4] that the choice of location be purely pragmatic (will people be there? if it is at sea, can I hitch a boat ride? is it an interesting place?).