Working with a Nautical Chart: Coordinates, Bearings, and Depths

On a paper map, you need to find a point with coordinates 46° 23.35' north latitude and 006° 23.4' west longitude. In another exercise, you need to mark off 7.4 miles along a bearing of 232° from the Point Victoria light. An error in fractions of a minute, the center of the symbol, or the position of the plotter will shift the result to the wrong part of the map.

Working with a nautical chart relies on two methods of plotting. The position is plotted using the intersection of latitude and longitude, or plotted from a known object using direction and distance. In both cases, an up-to-date map, accurate markings, and verification of the resulting point against the overall picture of the area are essential.

How to get started with a nautical chart

Nautical charts use a coordinate system to determine the position of a point on the Earth's surface. Latitude is measured from the equator: from 0° to 90° north or south. Longitude is measured from the Greenwich, or prime, meridian: from 0° to 180° east or west. The combination of these values determines the position.

The principle is easily illustrated using the example of New Orleans: its position is defined as 30° north latitude and 90° west longitude. If the first coordinate is placed north of the equator and the second west of Greenwich, the intersection point will show the desired location. On a map, the same principle applies to degrees, minutes, and fractions of a minute on the scales along the edges of the page.

Before plotting, check the chart's edition number, print date, and last update date against Notices to Mariners. The chart's coordinate system must match the GPS system. Map 3 in the training set has a scale of 1:100,000; Map 4 contains more detailed sections, but some of its sections are fictitious and not intended for navigation.

What publications and markings are needed before entering the area?

A map doesn't replace reference books. They help decipher symbols, prepare for harbor entry, and gather information about the chosen area. A ship's log records time, distance traveled, speed, compass heading, position, people on board, and activities performed.

• Symbols and Abbreviations Directory 5011 helps to read nautical chart symbols and obscure abbreviations.
• Almanac Contains information for the area, including information on ports, lights, signals, tides and tidal currents; it also provides information on marinas and available services.
• Pilot chart Provides information about ports, harbor entrances, hazards, as well as photographs and aerial photographs. It is read before entering the harbor.
• Tide tables show the tide heights in the places you plan to visit.
• Atlas of tidal currents shows the direction and speed of the tidal current.

An unknown symbol on a chart or in a sailing directions is checked against 5011 or the reference page of a school almanac. Before working with a particular area, it's important to ensure that the sailing directions, tables, and almanac are specific to that area.

How to read depths, map zero, colors, and dangers

Depths on the map in question are indicated in meters and are normalized to chart datum—a very low water level, roughly corresponding to the lowest astronomical tide. In the tutorial calculation, the actual depth is obtained by adding the chart depth to the high tide. The number in the circle, 27, means 27 meters, the number 3 and 6 means 3.6 meters, and 6 and 1 means 6.1 meters.

The chart datum distinguishes between deep water and dry land. The underlined number indicates the dry land height above the chart datum. The -0.2 m mark indicates an area that extends 0.2 m above the water at the chart datum. At a high tide of 0.2 m, it will be barely covered, and at a high tide of 1.2 m, there will be 1 m of water above it. This is a pedagogical example, not a complete calculation of the area's conditions.

Colors complement the numerical markings. Yellow denotes land, white and blue denote water, and the green area can be land or water depending on the tide level. As the water depth increases, the color changes from blue to light blue, then to white. The green area cannot be read in the same way as the permanently dry yellow area.

Heights and dimensions on the map are measured from the indicated levels. The height of the lighthouse in the example is 6 m above mean high water at spring tide. The clearance under the bridge is indicated as 2.6 m above the highest astronomical tide. The map also features rocks, sunken objects, anchorages, fairways, pipelines, ferries, high-speed vessels, surf, whirlpools, and traffic separation schemes. The meaning of specific symbols is determined by reference to a reference book: a cross with four dots denotes an object directly beneath the surface, and an anchor symbol denotes an anchorage.

How to determine coordinates on a nautical chart

To plot coordinates on a map, you'll need the map itself, a plotter, measuring instruments, and a sharp pencil. The side scale shows latitude, and the top scale shows longitude. In the assignment, you need to find the symbol at 46° 23.35' N, 006° 23.4' W. Latitude lies north of 46°, and longitude lies west of 6°.

Applying latitude

1. On the side scale, find 46° and count 23 minutes: 20, 21, 22, 23.
2. Add 0.35 minutes. After the 23.2 and 23.3 marks, the desired point will be halfway down the next division.
3. Make a small pencil mark. Place the plotter at a 90° angle to the side of the card and draw a line from the mark.
4. Look at the plotter from directly above: the line should be perpendicular to the side of the card.

Applying longitude and checking the point

1. On the top scale, find 6° West longitude, then count 20 minutes and the 23.4 minute mark.
2. Draw a line down from it. The intersection with the latitude line will give the desired position.
3. Check that both coordinates are measured in the appropriate fields on the map and that the lines are straight. Then compare the point with the overall view of the area.

In this example, the lines intersect at the "4 Juliet Delta" drilling platform in the upper left corner of the map. Plotting coordinates on the map depends on the map's current status, the accuracy of the scales, and the accuracy of its construction.

How to plot a position using bearing and distance

The second task starts from a known object. You need to determine which object is 7.4 miles along a bearing of 232° from the Point Victoria light. The light's position on the map is in the center of the star, not at the edge of the symbol. First, estimate the direction: 232° lies in the southwest sector, and the constructed line should look exactly like that.

Construction of a given bearing

1. Find the Point Victoria light and set the edge of the plotter exactly through the center of the star.
2. Set the rotating dial to 232°. The large arrow on the plotter should point in the direction of travel, and the small arrow should point north toward the top of the map.
3. Move and rotate the plotter until the disc lines align with the vertical or horizontal lines on the chart. Sometimes it's easier to position the disc over an area of seawater to make the arrows more visible.
4. Look from above and draw a line from the center of the fire in the given direction.
5. Using measuring tools, mark off 7.4 miles from the starting point along this line. The distance is measured using the map's side scale.

A latitude scale is used for measurement: one minute of latitude equals one nautical mile. On Map 3, each division of five equals 0.2 miles. The distance is measured with measuring instruments, transferred to the side scale, and counted off seven miles and another 0.4 miles.

Taking bearings and distances between points

1. Align the edge of the plotter with the center of the Point Victoria light and the center of the oil platform marker. If necessary, draw a thin line between them with a sharp pencil.
2. Make sure the large arrow points away from the fire and toward the platform. Rotate the central disk so the small arrows point north, and align its grid with the map lines.
3. Read the angle on the plotter's scale. In the example, the true bearing is 232°: directions on the map are measured relative to true north.
4. Place the feet of the measuring instruments in the centers of the two markings, transfer the opening to the side latitude scale and get a distance of 7.4 miles.

To determine a bearing on a nautical chart, compare the plotted line with the overall picture. If the line points in the opposite direction instead of southwest, the plotter may be upside down and the arrows may be oriented incorrectly.

What to check before completing the installation

Nautical navigation using a paper chart requires a sequence: first prepare and read the chart, then plot the position and compare it with the surrounding area.

Check the publication's relevance and the GPS coordinate system; distinguish between depth, dewatering, object height, and vertical clearance relative to the specified levels. Ensure the origin is taken from the symbol's center, the lines are drawn from the correct scales, the distance is measured using the latitude scale, and the bearing direction matches the overall picture.