COLREG - CPA / TCPA Calculator

Vector resolution of own and target velocity gives relative velocity. CPA is the perpendicular distance from own ship's relative track to target current position. TCPA is the time until the perpendicular foot is reached. Per COLREG Rule 7, risk of collision is determined by, among others, an unchanging compass bearing of an approaching vessel. CPA below 1.5 nm at moderate ranges typically indicates risk; CPA below 0.5 nm typically requires action; TCPA under 6 minutes indicates urgency.

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