Tidal diamonds
From Biocrawler, the free encyclopedia.
Tidal diamonds are marks on an Admiralty Charts used to calculate direction and speed of tidal streams.
The marks consist of a letter of the roman alphabet in a rhombus, printed in purple ink. On any particular chart each tidal diamond will have a unique letter starting from "A" and continuing alphabetically. Somewhere on the chart will be a table and the table will contain a grid of seven rows and three columns for each index letter. The rows will relate to the time before or after high water at a stated reference port and the columns will give the direction of the tidal stream and its speed during spring tides and neap tides.
NB it would be foolish to attempt navigation without proper training.
