Closest Point of Approach (CPA)

The predicted minimum distance between two moving targets if both maintain their present course and speed.

Commonly abbreviated to CPA, it is calculated from relative motion and is normally considered with Time to Closest Point of Approach, or TCPA. Radar, AIS and plotting systems can display these values to support assessment of an approaching vessel.

CPA is a prediction based on the data and motion at that moment. Target manoeuvres, own-ship manoeuvres, sensor error, AIS data quality and unstable tracking can change the result. A larger CPA does not by itself prove that a situation is safe, and collision risk must be assessed using all available information.