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Tidal range around the world of the principal lunar semi-diurnal constituent
(images2.imgbox.com)
More info about principal lunar semi-diurnal constituent. Basically it is the tide that the Moon causes twice everyday. The tide can be larger or smaller due to other factors, e.g. varying distance between Moon and Earth, or when the Sun creates spring or neap tide.
It's a map of the oceans, the pacific ocean is the center. The land is split for that reason