II.

Basic Tools

To determine their position on Earth, navigators first find their position on maps and nautical charts. They rely on a few basic tools to accomplish this task. Meticulously detailed maps and nautical charts help navigators to anticipate upcoming obstacles in time to change course and to visualize their location in relation to their destination. Latitude and longitude provide navigators with a unique numerical identifier for every point on Earth, much the way that street addresses identify individual buildings or houses. A magnetic compass indicates direction, helping navigators set and adhere to a course when other telltale reference points are absent. Plotting tools help navigators plot their progress as they travel.

A.

Maps and Charts

Landmarks and other references are displayed on maps and nautical charts, small paper or computer representations of the planet Earth and its parts. Unlike a round globe, which is a miniature of Earth itself, maps and charts are flat. The area that can be represented on a map or a chart depends on the map’s scale—that is, the ratio between the size of the map and the size of the area it covers. A map of a large area uses a small scale, and a map of a small area uses a large scale. A map of the entire world has a very small scale, for example 1:1,000,000, meaning that one unit on the map represents 1 million units on Earth’s surface. The scale of a map of a smaller area, such as the Great Lakes, may be 1:100,000. On a map of this scale, 1 unit represents 100,000 units. A unit may be a centimeter, an inch, or another measure.

As a rule, the less area the map shows, the more detail it includes. Some maps show only selected types of detail. A road map, for example, shows only the information that a driver needs, including roads, highways, towns, lakes, rivers, and mountains. A topographical map, used by hikers and mountain climbers, provides contour lines representing the exact height and shape of mountains, valleys, and other topographical features.

A nautical chart provides detailed information about a body of water, such as a lake, harbor, ocean, bay, or segment of coastline. Charts indicate water depth and characteristics of the sea bottom. They show the height and location of landmarks, such as islands or other landmasses, rocks, hills, radio towers, and buildings. Charts also provide detailed information about aids to navigation—lighthouses, buoys, and other artificial markers that help navigators avoid shallow water and other perils.

To help navigators locate their position at night or in foggy weather, many aids to navigation are equipped with lights or emit sounds or radio signals, or have both lights and sounds. Navigators can approximate their position by estimating the distance to an aid to navigation. The distance at which lighthouses and channel markers can be seen depends on their height, because Earth’s surface is curved. Someone standing in a small boat can see a light that is 1 m (3 ft) tall from 6 km (4 mi) away. A 15-m (50-ft) light can be seen from 14 km (9 mi) away, and a light 30 m (100 ft) tall is visible from as far away as 19 km (12 mi). A light’s brightness also factors in because bright lights shine farther than dim ones.

B.

Longitude and Latitude

Almost all maps and charts are designed so that north is at the top, south at the bottom, east on the right, and west on the left. In addition, maps and charts show latitude and longitude, a system of geometrical coordinates that provides a simple way to identify location. Navigators express their north-south position using parallels of latitude—lines running across a map, chart, or globe from left to right (west to east). A latitude coordinate indicates distance from the equator, which encircles the middle of the globe, dividing it into the northern and southern hemispheres. Navigators express their east-west position with meridians of longitude, lines running from top to bottom (north to south) on a map, chart, or globe. A longitude coordinate indicates distance from the prime meridian, which runs through Greenwich, England, near London.

Navigators express distance in terms of degrees (usually indicated by the symbol °). Parallels of latitude north of the equator, which lies at 0°, are identified as north, and those south of the equator are identified as south. The North Pole lies at latitude 90° north, the South Pole, at latitude 90° south. By international agreement, the prime meridian lies at 0° longitude. Meridians of longitude east of the prime meridian are designated as east, and those west of the prime meridian are identified as west. On maps, the designations are abbreviated as N, S, E, and W.

Each degree of latitude and longitude is divided into 60 minutes, and each minute is further divided into 60 seconds. Navigators measure distance in nautical miles, an internationally agreed-on standard equaling the average length of one minute of one degree, or 1,852 m (6,076 ft).

Like street addresses, position coordinates identify the latitude and longitude lines that intersect at a particular location. Houston, Texas, for example, is located at latitude 29°46' north, longitude 95°22' west, and Nairobi, Kenya, at latitude 1°17' south, longitude 36°49' east. Latitude and longitude prove particularly useful in describing locations in the middle of the ocean. On April 14, 1912 the ocean liner Titanic struck an iceberg in the northern Atlantic Ocean at latitude 41°33' north, longitude 50°01' west.

The relationship between longitude and time is also an important tool for navigators. Time is derived from the position of the Sun as it appears to move from east to west across the sky. Noon occurs when the Sun reaches its highest point in the sky—that is, the moment when the Sun has completed its rise but has not yet started to set. By international agreement, the time of day at the prime meridian (0°), called Greenwich mean time (GMT), sets the standard for the rest of the world. The Sun moves 15° each hour, making the time one hour earlier every 15° west of the prime meridian (see Time Zone). When it is 3 pm at Greenwich (longitude 0°), at longitude 75° west (near New York City) the time is five hours earlier (10 am), and at longitude 120° west (near Los Angeles, California) it is eight hours earlier (7 am). Where longitude 180° west meets longitude 180° east in the Pacific Ocean is the international date line. There, the date changes by one day. When it is 10 am Monday on the east side of the international date line, it is 10 am on Tuesday on the west side of the line.

C.

Compass

The magnetic compass is one of the oldest and most reliable navigation instruments still in use today. It always points north, providing navigators with a fixed reference point. Using a compass and a map or chart, a skilled, careful navigator can direct a craft from one destination to another, even in fog or at night.

The visible part of a magnetic compass includes a magnetized, round metal dial called the compass card, which is marked with a prominent N for north. Contained within a clear plastic dome filled with fluid, the compass card swings freely on a sharp point called the pivot. The magnetized compass card points north under the influence of Earth’s magnetic field (see Earth: The Core and Earth's Magnetism). Also rimming the compass card are letters for east, south, and west (called the cardinal points) and finer ticks marking each of the 360 degrees of a circle. These are encircled by as many as eight small posts, called lubber’s lines, which make the compass easier to read when a craft is underway.

Magnetic compasses do not point to true north—that is, the top of the axis around which Earth rotates, also called the North Pole. Instead, compass arrows point to magnetic north, the point where Earth’s magnetic field is the most concentrated (see Magnetic Pole). This point lies in northern Canada at approximately latitude 74° north, longitude 101° west—about 1,600 km (about 1,000 mi) from true north. The difference in direction between true north and magnetic north, called variation, differs from place to place and year to year because Earth’s magnetic field shifts over time. Most nautical charts graphically represent the directions to true and magnetic north, plus the local variation, in the form of a printed compass dial called a compass rose.

Any magnetic substance—for instance, the steel in a ship—can interfere with a magnetic compass and cause errors. Magnetic compasses must be regularly tested and compensated by experts called compass adjustors.

Large vessels and commercial craft usually use a gyrocompass, which incorporates a gyroscope, a spherical or disk-shaped device that rotates in one direction. A gyrocompass provides a fixed reference point because it rotates continuously in one direction, regardless of the movement of the craft. Gyrocompasses are not subject to magnetic errors because they point to true north and are unaffected by magnetism.

D.

Plotting Tools

Navigators use plotting tools to draw and record navigation information on paper charts. Every navigator needs a straightedge, such as a ruler, to draw a straight line between points. Dividers, two short metal legs joined at a hinge, help measure distances on the chart. A plotter enables the navigator to calculate and record compass courses on the chart. There are several types of plotters. Parallel rulers consist of two wooden or plastic straightedges linked by short, hinged arms. Parallel rulers are designed to transfer compass directions from a chart’s compass rose to other sections of the chart. Navigators move the parallel rulers—one straightedge at a time to ensure accuracy—across the chart to transfer courses and compass bearings from one place to another. Another type of plotter, called a course protractor, consists of a straightedge and an angle-measuring device called a protractor. Instead of the compass rose, course protractors use the chart’s longitude-latitude grid as a directional reference.

Today many navigators choose to work with electronic charts on computers instead of paper charts, and with electronic plotting tools. This computer software essentially mimics the function of traditional charts and plotting tools.