Galactography

The survey demarcates space in several zones, corresponding roughly to nearby galaxies and globular clusters. "Zone zero" represents the disc and core of the Milky Way, the barred spiral which contains almost all the stars in the galaxy. Nearly all atlas references are within this zone, so it is usually omitted when writing UGRs. The only other zone which humans are known to have reached is the galactic halo, a much larger volume around zone zero which contains scattered older stars. In theory there could be spacefold routes into the Large Magellanic Cloud (a third zone) and beyond, but no reliable reports of these have been recorded.

Volumes, systems, stars and planets

The volume of each zone is subdivided into some number of volumes, whose size depends on the exact zone. Within the disc, volumes are cubes 64 light years on a side. In the halo they are cubes 1024 light years in width. The simplest UGRs represent entire volumes. The zone is specified first (optionally for the disc) by writing it in square brackets. The volume designation comes immediately after. Usually this will be three hexadecimal numbers indicating the volume's relative position in space "xxx:yyy:zzz." For example, the centre of the galaxy is around volume 400:400:80. On the other hand, volume [01]85:34:92 would be in the halo.

Within each volume, star systems are numbered, with the star system separated from the volume by a dash. So a UGR of 302:517:23-16 means the 16th listed system in volume 302:517:23, and [01]85:34:92-5176 would be the 5176th system in that halo volume. System numbering is deliberately random, as a matter of policy. Rumour has it that Emperor John 14th decreed this, in a peeved reaction to endless pleas from planet governors who all wanted their systems to be listed first.

For each system, orbiting bodies are listed with another number appended to the UGR by a dash, counting planets and major planets first, then moons, asteroids and other bodies. In other words, they are organized by the body they orbit: first "children" of the star, then children of the children. So if our example system has 8 planets, 302:517:23-16-1 is the planet nearest to its star and 302:517:23-16-12 would be some other object, perhaps a moon.

Since most stars are in multiple-star systems, UGRs can also designate stars within a system. These are listed similarly to planets, only separated by a plus sign instead of a dash. So [01]85:34:92-5176+2 would be the second star in that system, and [01]85:34:92-5176-19 would be the 19th orbital. There is no way to tell from a UGR exactly which star a planet is orbiting, because it is possible for planets to orbit pairs and groups as a whole.

Spatial references

UGRs are not limited to identifying objects; they can also designate exact points in space or on orbitals, as well as areas on the surface of an orbital. An exact spatial reference is given by appending three numbers in the form "/x/y/z" to the UGR. These represent spatial offsets along those axes, in metres using Galactic Co-Ordinates, and are usually written in scientific notation. So, [01]85:34:92/1.4e10/0./1.4e10 would represent a point in space approximately 20 million km from the centre of the system. A reference [01]85:34:92-5176-19/42000/108000/752000 would represent a point several hundred km from the centre of an orbital (probably inside it).

These sort of exact spatial references are rare and mostly of use to scientists and surveyors. Much more common are "grid" references, which identify some subdivision of an orbital, typically an area of its surface. These are written similarly to systems and orbitals, by appending a single number to the UGR with a dash. The number must be looked up in an index from one of several complex but standard lists. For example, 302:517:23-16-1-0 would represent a single "grid point" on the first orbital of the 16th star in the volume 302:517:23 in the disc. Typically, grid point 0 indicates an area around the north pole of a planet.