Co-tidal Line

A co-tidal line is a line on a tidal chart joining all points at which the high water of a given constituent occurs at the same time, usually labeled in Greenwich phase lag or in lunar hours after the Moon’s transit. For a semidiurnal amphidromic system the co-tidal lines radiate from the amphidromic point, and the time taken for the high water to sweep from one line to the next shows the tide wave progressing around the basin. Read with co-range lines, which give the height, co-tidal lines let a navigator infer the time of high water at an unobserved position and underpin the construction of tide tables and tidal-stream atlases.