The Offshore Voyaging Reference Site

Q&A: Trans-Ocean Navigation

Question: We are crossing the Atlantic from the Canary Islands to St. Lucia. I found a large scale chart of the Atlantic (Imray Passage Chart 100). This single chart shows the entire Atlantic Ocean taking the earth’s curvature into account and therefore does NOT have a compass rose to help steer by. I am having trouble finding a series of smaller charts that DO have a compass rose that might be a bit easier to navigate by. Do you know of a source where I might purchase charts of a smaller scale?

Answer: The issue here is much deeper than that. No such charts exist. At sea, navigation was, before chart plotters, done on plotting sheets that are basically blank charts for each area of latitude. These sheets showed a true compass rose only. Each day the navigator looked up the variation on a passage chart and then set the compass course after factoring in the great circle course, if appropriate. To do this manually you need to understand lines of equal variation and the difference between great circle and Mercator (rhumb line) routing. These days many, probably all, chart plotters and GPS units will do all this for you automatically, although you still need to make overall routing decisions and understand the difference between a great circle and a rhumb line course.

In my opinion it is vital that before you set off on an ocean crossing, someone on your boat have a good understanding of these basic navigation issues. I would suggest a good book on navigation, or better still a navigation course. Otherwise, in case of an electrical failure, or the death of your plotter, you will be, very literally, all at sea.

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