Last night I accidentally stumbled on one of the best images of ocean swell that I have come across. I was looking at Essaouira on Google Earth and happened to notice that the image showed some really clear swell patterns.
In this image we can see the sets of swell arriving from the top-left of the picture (northwest). We can then see the distinct and different patterns that are formed as these swells interact with Mogador island off the coast of Essaouira. The swell diffracts through the gaps either side of the island and then these two patterns meet again in a thin line that runs from the top of the island to the land just above the large breaking wave.
Down at sea level it takes a lifetime to interpret these patterns, but it is so much easier to see how the great Polynesian and Micronesian navigators would have been able to find islands by interpreting these different motions in the sea, when they are seen clearly from above like this.
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For a complete guide to Natural Navigation read Tristan’s books.