Heading
In navigation, the heading was used to describe directions in space. Headings were programmed in at the ship's helm control using a combination of two 360- directions on planes that are at right angles to one another, separated by the word "mark". () 

The given heading can be used to determine the course or direction that a starship was actually moving. () The course itself could be used to measure distance traveled. () Starships traveling at warp speeds usually traveled in a straight line to the destination coordinates. () While sometimes, a course could also be a more elaborate flight path, that included changes in heading and parabolic trajectories. (; ) If a course was not set before engaging the warp drive, it was possible that one "could warp [them]selves into oblivion by crashing into a planet." ()

A more precise measuring angle system, beyond the 360-degree system, was sometimes used with headings such as 883-mark-41 or 902-mark-5. (; ; ; )

In June of 2151, Captain Jonathan Archer ordered a course correction as he picked a Vulcan monastery at P'Jem as a destination following his studies of the Vulcan star charts. Later, while being held hostage by the Andorian Imperial Guard at the  he lied to Thy'lek Shran when telling him that this planet was along their course and the landing party just came to visit the monastery. ()

Examples of headings 
 Heading from the Bajoran system to the Klingon Empire was 327-mark-215. ()
 Heading from Deep Space 9 to the Cardassian Empire was 269-mark-032. ()
 Heading from the Federation-Tzenkethi border to the Tzenkethi planet M'kemas III was 015-mark-47. ()
 Heading from Outpost 4 to the Romulan Neutral Zone was 111-mark-14. ()
 Heading from the Romulan Neutral Zone to Vulcan was 143-mark-012. ()
 Heading from Starbase 234 to the Klingon-Romulan border was 118-mark-357. () 
 Heading from Starbase 375 to the Cardassian border was 054-mark-093. ()
 Heading from Starbase 55 to coordinates 002/3845 was 223-mark-85. ()
 Heading from Tarsas III to Bynaus was 233-mark-45. ()
 Heading from the Vulcan system to the Laurentian system was 151-mark-3. ()
 Heading from Ivor Prime to Earth was 0-mark-215. ()
 In 2371,  was holding a course 351-mark-10 on its voyage back to Earth from the Delta Quadrant. ()
 In the more precise measuring angle system, heading from the Klingon-Federation border away from the Milky Way Galaxy was 902-mark-5. ()

Appendices

See also 
 Bearing
 Coordinates
 Grid
 Navigation
 Quadrant
 Sector

Background information 
The first use of headings in degrees occurred in  episode . The meaning of headings as two 360-degree angles was established in "Datalore". The system of degrees was further explained in reference books, such as Star Trek: The Next Generation Technical Manual (pp. 36, 37). 

The first 360-degree figure is a direction in an imaginary plane similar to the galactic plane, but between the ship and the center of the galaxy. What ever the location of the ship in the galaxy is, heading 000-mark-0 is always a direct course to the center of the galaxy. Heading 180-mark-0 is a direct heading away from it. A heading from Earth directly to the Beta Quadrant would be 090-mark-0 and further into the Alpha Quadrant 270-mark-0.

As explained in "Datalore", the second 360-degree figure is the elevation angle at a right angle to the previous plane. A heading of 360-mark-90 would be a heading directly upwards away from the plane, where as 360-mark-270 a heading directly downwards.

External link 
 

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