Why is it important to monitor CPA and TCPA together rather than separately?

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Understanding the risk of collision requires both how close you’ll get and when that closeness will occur. Close Point of Approach tells you the distance at the closest pass if both vessels keep on the same course and speed. Time to Closest Point of Approach tells you when that moment will happen. Together, they show not only how near the other vessel could come, but also how urgent the situation is, which guides timely actions like steering changes or speed adjustments.

If you look at distance alone, a small future distance might seem alarming but could be several minutes away, giving you time to act. If you rely only on the timing, you might miss how close the other vessel will actually get. They aren’t redundant pieces of information; they describe different, equally important aspects of the same encounter. The option about weather or wind isn’t relevant here, and saying only one of them matters ignores the need to plan actions based on both proximity and timing.