In approaching a strange harbor the chart should be scrutinized for prominent marks, and these identified as soon as possible, then the lesser objects can be picked up by their relative positions with the already identified landmarks. Among the lesser marks may be found cliffs, boulders, sandy beaches, vegetation, buildings (particularly church spires and houses with cupolas). Prominent elevations of land always serve to identify a locality. The chart shows these elevations clearly by contour lines.
A twenty-foot contour line, for instance, shows the line of the cut, should the hill be sawed off twenty feet above the sea. When contour lines are wide apart the land has a gradual slope, and as the grade becomes steeper the contour lines come closer proportionately.
The chart, wherever possible, represents the earth as seen from overhead, but in the case of vertical objects, they are of necessity shown horizontally. One of the notable cases of this is in the representation of cliffs, which in order to show their height are drawn with the side view as seen from the water.
Numbers seen on the land show the height above the high water.
From the topographical features of a chart we turn to those of hydrography. All the depths indicated on a chart are those existing at mean low water, on the Atlantic Coast, and mean lower low water on the Pacific Coast.
All the British Admiralty and most of the Hydrographic Office charts are reduced to the level of low water at ordinary spring tides. While there is usually more water to be expected than shown, it must be remembered that when the plane of mean low water is used, the low waters that, roughly speaking, come between the moon’s first and last quarters, will fall below the soundings on the charts.
The effect of an abnormal barometer and high winds must at times be borne in mind, for a continuous northwest wind will make a vital difference in the depths along the Atlantic seaboard, especially in the Delaware and Chesapeake Bays.
Very often there is information that can not be symbolized on the chart and is placed in italics in the form of a note. These are always important and should be read carefully.