Dip
Dip is the difference in elevation angle the line of sight to an object from one vantage point (say from a director) and that from another vantage point (say a gun mounting on the deck).
The Royal Navy defined a "datum point" (often, the armoured director position, or the trunnions of the turret highest above the water) for each ship which would share precise gunnery pointing angles, as in director firing, and provide mechanisms that would convert angles from that position to their own. This permitted the director's elevation transmitters to send out these reference elevations, confident that each receiver would massage the angle a bit as their own position on board required.
For instance, the Royal Navy's elevation receivers often featured a "dip strip" that could be shipped on their face. These corrected a number of small issues, but chief amongst them was that which earned them their name.
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