At some important ports information as to the state of the tide is given to vessels, either by means of signal balls, or by automatic tidal indicators, as at the Narrows in New York Harbor, where a large dial shows to passing vessels the height of the tide, and an arrow indicates whether it is rising or falling.
The tidal information becomes important and must be considered in navigation or in anchoring in waters where the available depth at low water approximates the draft of the vessel. In the general use of coast charts it is also important to observe the effect of the stage of tide on the appearance of many features. Rocks rising some feet above low water may be entirely submerged at high water. In some areas the aspect may be radically changed between high and low water by the baring of extensive shoals or reefs.
Currents. Information, when available, as to currents is given either by a note or by current arrows placed on the chart at the position of observation. Additional information as to certain regions is given in the United States Tide Tables. Tidal currents, flood and ebb, and currents not due to tidal action are distinguished by symbols, and the velocity is given in knots, and on some charts is indicated by the lengths of the arrows.
Complete and systematic current observations have been made in comparatively few localities because of the time and expense necessary to get the full information as to the variations of the currents with the tides and seasons. Ordinarily therefore the current arrows shown on charts indicate only the average direction and velocity, or possibly only the conditions existing at the season when the survey was made. Oceanic and coast currents are probably much less uniform than might be inferred from the current streams drawn on maps and charts. A more systematic investigation of ocean currents is required to fulfill the needs of navigation.
The tidal currents seldom turn with the tides, and there may be an interval of as much as three hours between the time of high tide or low tide and slack water. This leads to the apparent anomaly that in cases the current may be running with its greatest velocity at the time of high or low water, and may be running into a channel for several hours after the tide commences to fall. It is therefore, evidently, not safe to draw inferences as to currents solely from the tidal heights.
There are passages where the tidal currents become of the greatest importance to navigation, as, for instance, in Seymour Narrows on the inside route to Alaska, where the current velocity reaches 12 knots and the interval of apparent slack water lasts but a few minutes.
Elevations. The unit used for elevations is also stated on the face of the chart, as also the plane to which elevations are referred. On the United States charts this is generally mean high water and on British charts the high water of ordinary spring tides. Rocks and islets usually have figures shown beside them, either in brackets or underscored, which indicate the height above high water. Rocks which are bare at low water sometimes have a note "dries" or "bares" so many feet, indicating their height above low tide, although they are covered at high tide. The British charts in some regions where there is a large range of tide have underlined figures in the area between high water and low water indicating the heights above low water, or the depths of water over the bank at high water, as explained in each case.
Topography. The land area on most charts is distinguished from the water area by a stipple or tint; on some charts the topographic features have, however, been depended upon to bring out the land from the water. The solid shore line is the high-water line, and should be clear on the chart; the area between high and low water is sanded or otherwise shaded on all charts. The relief of the land is represented by hill shading or by contour lines which are the successive curves of elevation on the land. Topographic symbols are used for some of the more important features, such as cliffs, rocky ledges, buildings, bridges, trees, roads, etc. It is important for the navigator to understand the significance of the hill representation and the symbols, as they will aid him in recognizing a coast or island, and in identifying landmarks.
Date of survey and publication. There is usually an authority note on each chart showing the source of information or date of survey; if on a coast subject to change, the latter is important. On the United States Coast Survey charts the date of publication of the edition is given, and on British and other charts the date of both large and small corrections. The chart catalogues give the dates of the last editions, or the dates of extensive corrections, and this affords a means of seeing whether the copy of the chart in use is the latest edition available.