How to find latitude and longitude on a nautical chart

Any GPS receiver will find the latitude and longitude along its navigation routes at any time. But as a skipper, have you plotted this information on a nautical chart to check your position? Take your sailing skills to the next level with this vital sailing skill!

Navigating with Latitude

Cartographers create a grid-like network on their navigation chart. Lines of latitude run horizontally. The lines of longitude run in the vertical direction. Imagine the earth, balanced on its axis without tilt. Wrap a “belt” around the earth, divide it in two, and you have the equator, the birthplace of Latitude. Label the equator 0 degrees.

Lines of latitude are parallel to the equator to the north or south. To plot latitude to find out where you are, measure how many degrees you are north or south of the equator. Latitude peaks at 90 degrees at the north and south poles. Always label Latitude N, if it is north of the equator, or S, if it is south of the equator.

In chart navigation, use the scales on the right or left side to find your Latitude. These scales are divided into degrees, minutes, and tenths of a minute, or degrees, minutes, and seconds. One degree of latitude equals sixty minutes; one minute of latitude equals sixty seconds. Here’s a simple way to remember this:

1 degree = 60 minutes.

1 minute = 60 seconds.

Sailing with Longitude

Go back to your imaginary balloon. To measure Longitude, divide the land in half again, but this time lengthwise. Locate Greenwich, England on your globe. Draw a line around the earth that crosses Greenwich and the north and south poles. Cartographers call this Greenwich, or Prime Meridian, Longitude’s birthplace. Label the Greenwich meridian 0 degrees.

The lines of longitude are parallel to the vertical meridian of Greenwich to the east or west. To find longitude, measure how many degrees it is to the east or west of the Greenwich meridian. Longitude peaks at 180 degrees on the other side of the earth, on the international deadline. You must label Longitude E, if it is east of Greenwich, or W, if it is west of Greenwich.

Use the top or bottom of the graph to measure the length. Like latitude, longitude is divided into degrees, minutes, and tenths of minutes or degrees, minutes, and seconds.

How to convert minute increments

All nautical charts show minutes divided into increments so you can plot parts of a minute. For example, if your GPS position shows 23-13N; 82-16W, no increments to worry about. But, if your GPS position shows Latitude 23-13.278N; Length 82-16.786W, has minute increments. Before plotting your position, round the increments to the nearest tenth of a minute. Round like this: Latitude 23-13.3N; Length 82-16.8W.

Look at the Latitude scales (right or left side) and Longitude scales (top or bottom) on your graph. Are the minutes divided into tenths or seconds?

Some graphs show degrees, minutes, and tenths of a minute. The minutes will be divided into 10 small segments. Each small segment equals one tenth of a minute. Other charts show degrees, minutes, and seconds of a minute. If your graph shows degrees, minutes, and seconds, you will need to multiply the “tenths” of a minute by 6. Follow this example:

GPS reading (with minutes rounded as described above): Latitude 23-13.3N; Length 82-16.8W.

Multiply the latitude minute increment like this: 3 X 6 = 18 seconds.

Multiply the minute increment in length like this: 8 X 6 = 48 seconds.

Graph: Latitude 23 degrees, 13 minutes, 18 seconds; Longitude 82 degrees, 16 minutes, 48 ​​seconds.

How to plot latitude and longitude

Use a pair of dividers to plot your position by latitude and longitude in the table. Read the degrees and minutes from your GPS. Find the closest degrees and full minute of latitude on your GPS.

For example, for Latitude 23-13.3N, you would look for 23 degrees, 13 minutes on the left or right side scales on your navigation chart. Push a point from your dividers in the 13 minutes. Open the other leg 3 small segments (three-tenths), above the 13-minute mark. If your chart shows seconds instead of tenths, open the other leg of your divisors 18 seconds (.3 X 6) above the 13-minute mark. Place a pencil mark where the dividers touch the exact latitude.

Then graph its Length. Use the exact same method to plot its length. Be sure to use the scales in the top or bottom graph to plot your Length. When you have found your Length, place a pencil mark where the dividers touch the exact Length.

Find your exact position

Align your parallel rulers or any other rulers so that the top long edge touches the Latitude pencil mark. Make the parallel rulers or the perpendicular ruler so that when you draw on the latitude line, it is parallel to all other latitude lines. Draw a light pencil line along the body of the graph to a location close to where you marked the upper or lower length scale.

Repeat this same method to align and draw on your Longitude line. Where the longitude pencil line intersects the latitude pencil line shows your exact position. Now you should have a cross. Place a point where the two lines intersect. Circle the point. Erase the light lines drawn on the edges of the graphic to clean the graphic and keep its plot neat.

Follow these simple steps to quickly find latitude and longitude on your nautical chart. With these navigation skills, you will be on your way to becoming a safe boating skipper, wherever you choose to surf!