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.