This 'Meddo' sea chart from the Marshall Islands, where navigation was, and still is, an essential survival skill, was a closely guarded secret and only passed on within a small number of families. In order to memorise the main navigational features between the islands, sailors built special maps with sticks, bres and shells, to describe the archipelagos. They were created for personal use, and only the sailor who built them could wholly understand their meaning. Consultation would happen exclusively on land, as once at sea the sailor could not show any kind of uncertainty in the route, as that would have questioned his capacity and consequently undermined his social status.

These types of maps, apart from indicating the positions of the islands (through cowry shells), show the main characteristics of the sea. Marshall Island culture, aware of the fact that the presence of the islands altered the orientation of the strong ocean currents, profited by this knowledge to navigate in the archipelagos.

The Marshall Islands maps, being the first document to give information on currents and also on the geography of the islands, represent a milestone of nautical cartography.

There are three distinct categories:

1. Mattang or Wappepe: an abstract graphic used to teach the principles needed to read how the islands stop waves and currents. It illustrates different kinds of swells, and is not a chart of a specific place or area.

2. Rebbelib Map: contains the same information as the Meddo map, including most or, in some cases, all of the relative positions of the islands of one or both the chains of archipelagos, and is therefore most similar to a traditional western map or chart.

3. Meddo Map: indicates the real position of the islands, the direction of the main currents, the way in which the currents curve around the earth and how they intersect between themselves. It shows the distance from which it is possible to see a precise island. The Meddo maps present a side of one of the two main chains of archipelagos.

Marshall Islanders navigate by paying attention to the seasonal position of the stars, the wind, and the effect that the wind, and the shape and proximity of the islands, has on waves and currents. They do not rely only on visual clues, but on the 'feeling' of the waves, and the way that their boats respond to changes. Since the Marshall Islands are at the boundary between the easterly trade winds and the doldrums, easterly swells from the trades, northerly swells from the Pacifc and even westerly swells from storms bear down on the islands, depending on the season. These swells are interrupted by the islands, and the effect that has on the waves surrounding them, is what the islanders learn and use to determine their position between them.

The Marshall Islands are part of 'Micronesia' in the Pacifc, which also includes the Marianas, the Carolines, Kiribati, and Tuvalu. Their unique navigational techniques used by their inhabitants came to the attention of the western world at the end of the nineteenth century, when Captain Winkler published 'On Sea Charts Formerly Used in the Marshall Islands, with Notices on the Navigation of These Islanders in General', Smithsonian Report, 1899.