Galactography

The Universal Grid Reference system (UGR) allows us to describe any point in the galaxy, from entire quadrants to micron-sized specks of dust. It has seen use since the first Imperial Survey thousands of years ago, and understanding it is vital to the career of any spacefarer.

The Milky Way

Our galaxy is a "barred spiral" type, a flattened disc with a central bulge. It is roughly 100,000 ly across and averages about 1000 ly thick, although this is far from uniform and the disc is far from flat. Although practically the whole volume of the galaxy is populated with stars, many of them are of types not useful to humans and these have mostly been omitted from the Survey star catalogue.

Althogh the galaxy is round, standard rectangular coordinates are used to mark its volume of space. The x axis runs through the central bar, y axis 90 degrees to it with both in the average plane of the disc, and the z azis is oriented at 90 degrees to both, pointing "up" and "down" out of the galaxy.

Quadrants, systems, planets and grid references

The Survey covers a large volume of space enclosing the galaxy at its centre. This volume is divided into cubical quadrants 64 light years on each side, with 2048 along the x and y axes and 32 along the z axis. They are traditionally written in hexadecimal as 3 digits separated by colons "xxx:yyy:zz". For example, the centre of the galaxy is around quadrant 400:400:10. The survey also specifies a method for mapping quadrants ouside the normal volume of the galaxy - these are numbered and can be indicated with their number in brackets. For example, [1]002:003:25 would be a quadrant in some volume of space outside the galaxy.

Within each quadrant stars are listed in the order in which they were surveyed. So a UGR of 302:517:23-16 means the 16th listed star in quadrant 302:517:23. For each star, orbiting bodies are listed similarly, counting planets first and them moons, asteroids and other bodies. So if our example star has 8 planets, 302:517:23-16-1 is the planet nearest to the sun and 302:517:23-16-12 would be some other object, perhaps a moon.

Instead of a planet one might also see a spatial reference of the form r/theta/phi, where the coordinates are relative to the galactic plane and a line between the centre of the star and the galactic centre. Since stars move with time, these references are of relatively little accuracy and should be taken to mean a rough volume of space. Typically they are used to indicate a starting location for a search.

On planets, locations are given as "grid references". Each of these is an index in a particular type of grid used to map the planet. The grids are typically icosahedrons (12-sided figures) subdivided into many cells and projected into a sphere, so the cells divide the planet's surface into small and roughly equal areas. The index numbers themselves follow little obvious pattern but 0 is always the north pole of a planet and 11 is always the south pole. Other sorts of grids are listed in the survey guide but opportunities to use them are extremely rare.

UGRs might also include a fine reference, which is a set of three numbers giving the spatial offset from the centre of a particular grid cell. Fine references are written at the end of the UGRas #xyz, where each of x, y and z is a number written with its sign.

UGRs can represent entire quadrants, systems and planes, as well as points, with varying degrees of relativity. For example:

• 205 - means grid reference 205 on a planet specified already
• 5-10217 - grid reference 10217 of planet/moon/asteroid 5 in some system
• 5093-12-0 - the north pole of planet/moon 12 in star system 5093 of a quadrant
• 102:5A3:0F - a quadrant
• 3A2:001:1B-15 - star system 15 in quadrant 3A2:001:1B
• 059:12A:04-6702-3 - planet 3 of star system 6702 in quadrant 059:12A:04
• [0]102:093:07-8715-2-4109#+10257.203-52104.509-2.789 - the most complex possible UGR, a particular point in grid reference 4109 of planet 2 in system 8715, quadrant 102:093:07, galaxy 0 (the Milky Way).