Quote:
Originally Posted by Delgard
Like in doing a back azimuth from a known point. So many known points out in the middle of the Atlantic.
 |
I understand you were just trying to be funny. But, just to ensure there is no confusion on the part of anyone reading this, I feel I should explain:
The CPA or closest point of approach is not a specific point relative to the ocean. It is the point at which two ships will be closest to each other relative to their respective positions, speeds and courses. The CPA is described by a distance and bearing from each ship.
It is the job of the navigator (and ultimately of the captain) to know exactly where other ships and objects are in relation to their own ship in order to avoid a collision. Calculating a CPA is the first step in determining if a danger of collision exists. It is one of the easiest tasks to perform on a maneuvering board. For example: if the line drawn through successive plots leads directly to the center, you are on a collision course with the object being plotted.
In fact, you don't even need the maneuvering board to realize this. If the bearing of an object does not change over time, but the range continually decreases, then the CPA is 0 yds. In other words: you are going to collide. It's really just common sense.
And actually, there
are many known points in the middle of the Atlantic. Technically there are an infinite number of them. For example: 27°31.7' N, 43°12.1' W. The main job of the navigator is to determine at which of these points the ship currently is. Modern tools such as GPS and ECS make this job a snap. But more traditional methods such as celestial navigation at sea and bearings via pelorus or compass and soundings during coastal navigation were good enough to get the job done for hundreds of years.
However, it is best practice to use
all available sources of information to determine one's position. Only a fool would go to sea without a GPS today, relying only on celestial navigation. And, more than one accident has been caused by complacency and over-reliance on a GPS or ECS which may be inaccurate for various reasons. Information from one source should be continually cross-checked with information from another. Sometimes it's even as simple as lifting one's head from the glowing display and looking out the window.