III.

Map and Chart Projections

The initial planning and the end results of navigation are plotted on maps and charts. The nearly-spherical surface of the Earth is represented, or projected, on a plane surface upon which are superimposed the coordinates of latitude and longitude and also the desired features of surface and underwater topography. Charts are maps that accentuate the determination of position, direction, and distance, and stress points of interest to a navigator. Because no part of a sphere can be spread out flat without distortion, several projections have been developed to accommodate the navigator. Each projection has its own advantages and limitations and fulfils a specific need of the navigator.

The most widely used projection in navigation charts is the Mercator, named after the 16th-century Flemish mathematician and geographer Gerardus Mercator, who devised it. These charts portray the Earth's surface projected on a cylinder tangent to the surface of the Earth at the equator. When this cylinder is flattened out, the meridians, or lines of longitude, appear as equally spaced vertical lines and the parallels of latitude appear as parallel horizontal lines, spaced farther apart towards the poles. The Mercator projection, despite its great distortion, is popular because a course that follows a constant bearing—that is, makes a constant angle with the direction of north—is represented by a straight line on the chart.

A navigator usually attempts to find the shortest route between two points; this can be accomplished by following a great-circle course. The great circle between two points on the surface of the Earth represents the arc of a plane intersecting the sphere at its centre and is the shortest path on the spherical surface. Great-circle courses can be determined directly from great-circle charts, but because it is impracticable for a ship to travel on an ever-changing bearing, the usual practice is to follow a series of chords that approximate the great circle. These chords are normally plotted on a Mercator chart.

Most of the navigable waters of the world have been surveyed accurately by the hydrographic services of the principal maritime nations so that reliable charts of the waters are usually available to the navigator.

The hydrographic services of various countries also publish almanacs and sailing directions to assist the navigator. Sailing directions are descriptive books containing detailed information on coastal waters, harbour facilities, navigation aids, winds, tides, currents, dangers to navigation, directions for approaching and entering restricted waters, and other data that cannot be shown on the chart of the area. Similar information is provided for aircraft, with indications of features, such as ground topology and airways, that are a function of altitude.