What is the Closest Point of Approach (CPA) and how is it used in practice?

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What is the Closest Point of Approach (CPA) and how is it used in practice?

The distance of closest approach is the minimum distance two vessels will have to each other if they continue on their present courses and speeds. In practice you look at the relative motion between your ship and the other vessel and project forward to the moment when their separation is smallest. If that minimum separation is less than your safe-distance threshold, you know there’s a collision risk and you should take action (change course, slow down, etc.). This concept is usually considered together with the time to CPA, which tells you how soon that closest approach would occur. So the correct idea is that CPA is the smallest separation during the relative motion and it’s used to decide whether to take action to avoid collision. It’s not about when vessels are merely aligned along a bearing, not about how far you must travel to avoid collision, and not about any distance after a collision.

The distance of closest approach is the minimum distance two vessels will have to each other if they continue on their present courses and speeds. In practice you look at the relative motion between your ship and the other vessel and project forward to the moment when their separation is smallest. If that minimum separation is less than your safe-distance threshold, you know there’s a collision risk and you should take action (change course, slow down, etc.). This concept is usually considered together with the time to CPA, which tells you how soon that closest approach would occur.

So the correct idea is that CPA is the smallest separation during the relative motion and it’s used to decide whether to take action to avoid collision. It’s not about when vessels are merely aligned along a bearing, not about how far you must travel to avoid collision, and not about any distance after a collision.

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