Navigating by swells

The Pacific-island gridded navigation stick-charts are intriguing. How do they "work"? Not like the maps and charts we know.

These memory aids are an individual's interpretation of knowledge of the swells in the ocean - which are affected by the islands nearby. They are used only on land, often to instruct novices. Read more here and here. The one shown is from the Marshall Islands, north-west of Papua New Guinea.