Tidal Diamonds and Stream Atlases

A charted tidal diamond identifies a position whose table gives set and rate, commonly from six hours before to six hours after high water at a named reference port. Set is the direction towards which the water flows, expressed in degrees true; rate is speed in knots. Mean spring and neap rates are normally shown separately.

A tidal-stream atlas shows the pattern over a wider area, usually as one chartlet for each hour relative to the same reference high water. The arrow shows direction and its figures or key show spring and neap rates, often in tenths of a knot.

The reference is a clock for the stream diagram, not a claim that the stream turns at local high water. Confirm the reference port, its predicted HW time, the page's hour label and the applicable rate before using the vector in course-to-steer or estimated-position work.