Heading is an expression of the direction a space vessel is to travel in. It is composed of two numbers: an azimuth (horizontal) value and an elevation (vertical) value, separated by the designator "mark". Each number ranged between zero and three hundred sixty degrees. (TNG: "Datalore")
Heading differs from bearing in that heading describes the direction of travel, whereas bearing refers to the position of an object relative to the position of the vessel taking the bearing.
In 2355, Captain Jean-Luc Picard of the USS Stargazer ordered a sensor bearing of a hostile (later identified as Ferengi) ship), and was advised the bearing was seven-mark-nineteen (7-19), putting the vessel slightly above and to starboard of the Stargazer. He then ordered his helmsman to engage warp nine on a heading of seven-seven-mark-twenty (77-20), traveling further starboard, and up on the vertical plane. (TNG: "The Battle")
Some headings (or bearings) can be expressed using one of two different measurements. Turning completely around (or an object directly behind) could be expressed either as one-eight-zero-mark-zero (180-0) or zero-mark-one-eight-zero (0-180).
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