What is the difference between relative bearing and true bearing in ARPA plotting?

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This distinguishes two ways of describing where another vessel is on ARPA: relative bearing and true bearing. Relative bearing is the angle to the target measured from your own ship’s heading (your forward direction). True bearing is the angle measured from true north on the chart. ARPA can convert between them because it uses your ship’s heading and adds or subtracts accordingly. The conversion is straightforward: true bearing equals your heading plus the relative bearing, wrapped to 0–360 degrees. For example, with a heading of 60 degrees and a target at 20 degrees relative, the true bearing is 80 degrees. If heading is 350 degrees and the relative bearing is 20 degrees, the true bearing is 10 degrees (wrap around). Relative bearing stays the same from your ship’s perspective as you maneuver, while true bearing shifts with your heading; ARPA handles the conversion to keep plotting consistent with true north.