# Finding shipping lanes using a GPS receiver

To the editor:

For a singlehander, shipping lanes mean redoubled watchfulness, but how does he know when he is approaching one of them? In John Letcher’s book Self-contained Navigation, the author describes a method using the navigation tables to pinpoint where a great circle course between two harbors will be in a given area. With my innate proclivity to make calculation mistakes, this method was too complicated, so I devised a simpler one that makes use of a GPS receiver.

The course indicated by the GPS to a given point from your present position can be thought of as the initial course of a great circle. If you are aiming for the north pole, the GPS will give you a course of 360° true. If, on the other hand, you aim for the south pole, it will give you a course of 180° true. The difference between these two courses will be 180°. The same holds true if you are on the Equator, and going due east or west: 270° – 90° also comes to 180°. I considered that the same would be true of all great circle courses and that, by entering the coordinates of ports of origin and destination of commercial shipping, I could tell when I was approaching a shipping lane by noting how close the difference between the two courses given to me by the GPS came to 180°.

Some proof of the method’s validity came toward the end of my run from the Canaries to Trinidad on board my boat Pampero IV early this year. The Spanish yacht Cybeles, who was about one week ahead of me on the same course, warned me to be careful when approaching 15° N, 50° W as they had met three ships in that area. I checked on my pilot charts and it didn’t make sensethe indicated shipping lane from New York to Cabo Sao Roque (the NE tip of Brazil) came nowhere near that point. It was only by various iterations that I came to the truth: 15° N, 50° W was actually on a course from a point about 100 miles East of Cape Hatteras to Cabo Sao Roque.

The successors of Lt. Maury hadn’t taken into account changes in traffic: The Norfolk area has displaced New York as the main East Coast harbor.