In addition to its accurate representation of the configuration of the coast, a chart supplies the mariner with a variety of necessary information. The signs and abbreviations employed are usually explained in the corner of the chart. The following are the most important among these:—
The small numbers on a chart indicate the soundings, either in fathoms or feet, at low water ordinary spring tides. The nature of the bottom is also shown, the following being some of the abbreviations used:—cl., clay; c., coarse; f., fine; g., gravel; h., hard; s., soft; m., mud; oz., ooze; peb. pebbles; sh., shells.
The Roman numerals on the chart give the Tidal Establishment of the place; that is, the hour of high water at the full and change of the moon. Thus H. W. F. & C. XIh. 40m., signifies high water at full and change at eleven hours forty minutes. From this one is able by the following method, without any reference to tide tables, to calculate with sufficient accuracy the hour of high water. Find in your almanac the date of the last full or new moon, and for every day that has elapsed since then add forty-eight minutes to the time of high-water at full and change, as given on the chart.
Arrows on the chart indicate the direction of tides and currents: on some charts an arrow feathered on one side shows the direction of the flood-tide, an unfeathered arrow that of the ebb.
