Relative vectors vs true vectors: which statement is correct?

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Multiple Choice

Relative vectors vs true vectors: which statement is correct?

Think of it in terms of motion references: true vectors are the actual movement of a vessel over the Earth—its course over ground and speed over ground. They tell you where the vessel is going in the real world, which you compare to COLREGS to judge if a contact is crossing, overtaking, or head-on. Relative vectors, on the other hand, describe how another target appears to move from your own ship’s position and heading. They’re the motion of the target in your ship’s frame of reference, which is what you use to assess collision risk and to compute CPA (closest point of approach) and TCPA (time to CPA). ARPA uses both: the relative motion data helps you gauge how close and when a target may affect you, while the true motion data helps you determine the type of situation and what maneuver or action is appropriate under COLREGS. That combination is why the statement aligns with how these vectors are used in practice.

Think of it in terms of motion references: true vectors are the actual movement of a vessel over the Earth—its course over ground and speed over ground. They tell you where the vessel is going in the real world, which you compare to COLREGS to judge if a contact is crossing, overtaking, or head-on.

Relative vectors, on the other hand, describe how another target appears to move from your own ship’s position and heading. They’re the motion of the target in your ship’s frame of reference, which is what you use to assess collision risk and to compute CPA (closest point of approach) and TCPA (time to CPA).

ARPA uses both: the relative motion data helps you gauge how close and when a target may affect you, while the true motion data helps you determine the type of situation and what maneuver or action is appropriate under COLREGS. That combination is why the statement aligns with how these vectors are used in practice.

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