Safe Boat Operations – Nautical Chart
The nautical chart is one of the mariner’s most useful and most widely used navigational aids. Navigational charts contain a lot of information of great value to you as a boat Captain.
For the purpose of coastal navigation, the earth is considered to be a perfect sphere. To represent the features of the earth’s spherical surface on the flat surface of a chart, a process termed “projection” is used. The primary type of projection used in making piloting charts is called Mercator projections.
A Mercator projection is made by transferring the surface of the globe (representing the earth) onto a cylinder.
The equator is the reference point for accomplishing the projection from one geometric shape to another. The distinguishing feature of the Mercator projection is that the meridians are projected so they appear to be equal distance from each other and parallel.
Nautical charts usually have one or more compass roses printed on them. These are similar in appearance to the compass card and, like the compass card, are oriented with north at the top. Directions on the chart are measured by using the compass rose. Direction is measured as a straight line from the center point of the circle to a number on the compass rose. Plotting the direction and explanation of the terms is discussed later.
True direction is printed around the outside of the compass rose. Magnetic direction is printed around the inside of the compass rose. An arrow points to magnetic north. Variation, the difference between true and magnetic north for the particular area covered by the chart, is printed in the middle of the compass rose (as well as any annual change).
One of the more vital services a chart performs is to describe the bottom characteristics to a boat operator. This is accomplished through the use of combinations of numbers, color codes, underwater contour lines, and a system of symbols and abbreviations.
The nautical chart water depth is measured downward from sea level at low water (soundings). Heights or landmarks are given in feet above sea level. In the interest of navigation safety, the mean, or average, of the lower of the two tides in the tidal cycle is used for soundings. Most of the numbers on the chart represent soundings of the water depth at mean low tide. Datum refers to a base line from which a chart’s vertical measurements are made. Generally, shallow water is tinted darker blue on a chart, while deeper water is tinted light blue or white.
Contour lines, also called fathom curves, connect points of roughly equal depth and provide a profile of the bottom. These lines are either numbered or coded, according to depth, using particular combinations of dots and dashes. Depth of water may be charted in feet, meters or fathoms (a fathom equals six feet). The chart legend will indicate which unit (feet, meters or fathoms) is used.