The science of mapping interstellar space. Basic to any science of mapping is a coordinate system. The scientific system used by the Imperium is based on rings of longitude, rays of latitude, and parsecs. By convention, rays of latitude and rings of longitude are measured from a starting point at Reference/Core (0140), the world is used as a baseline and is where the data from the First and Second Imperial Grand Surveys are stored.
This mapping system is highly Imperium-centric, and other systems are used by other peoples and races outside the Imperium. This system has gained wide acceptance, however, among Imperium dominated client-states, human and otherwise. This mapping system breaks down and is prone to error beyond certain limits. It does serve admirably for a band approximately 400 parsecs wide at a longitude of 10,000 parsecs, which is a region precisely centered on the Imperium.
In ordinary discourse, a world is referred to by its name and sector then parsec/hex in parentheses. For example, the homeworld of Archduke Norris is Regina/Spinward Marches (1910) that is, the world Regina is in hex 1910 in Spinward Marches Sector.