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In Greyhound (2020), from Fletcher-class destroyer, Commander Ernest commands Taker & Helmsman to intercept German U-boat, which was following 37 Allied ships, which are making its way to Liverpool:

Commander Ernest: Report all bearings as relative.

Helmsman: All stations, Bridge. Report all bearings as relative.

What does "relative" mean in naval context?

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In navigation, a relative bearing is an angle measured clockwise from the heading of the boat. So 000⁰ is straight ahead and 090⁰ is on the right. It contrasts with absolute bearings which are measured relative to North (so 090⁰ is due east).

https://en.wikipedia.org/wiki/Relative_bearing

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  • Exactly. This is a much quicker way to pinpoint the location of a target. Everyone onboard the ship will know the direction of the bow relative to their personal position. They may not know the direction of true or magnetic North. Nor, would they have time to shoot a compass azimuth when trying to spot a diving submarine or an incoming torpedoe. But, they can give a general warning based on a 360° arc around the ship. Similar to using clock direction.
    – Dean F.
    Jul 20 '20 at 15:11
  • On the other hand, it's useless for describing the current direction of travel.
    – James K
    Jul 20 '20 at 16:35
  • Correct. Although, aviation still uses relative bearing when navigating to non-directional beacons and other fixes. Just not as much in the US in recent years. Modern technology has made it all but obsolete. Incidentally, in both land-based military navigation and aviation, using the clock method Is always based on your true direction of travel. The 12 o’clock (0/360°) position is the direction in which you are moving regardless of which direction you are actually facing. This is mainly used in traffic/obstacle avoidance and movement to fire.
    – Dean F.
    Jul 20 '20 at 17:05

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