How to Find Latitude and Longitude on a Nautical Chart

Any GPS receiver will find Latitude and Longitude along your sailing routes at any moment. But as a skipper, have you plotted this information onto a nautical chart to check your position? Boost your sailing navigation skills to the next level with this vital sailing skill!

Navigating with Latitude

Cartographers create a grid-like web on your navigation chart. Latitude lines run in a horizontal direction. Longitude lines run in a vertical direction. Imagine the earth, balanced on her axis without a tilt. Wrap a "belt" around the earth, divide it in two and you have the equator – birthplace of Latitude. Label the equator 0 degrees.

Latitude lines parallel the equator to the north or to the south. To plot Latitude to find out where you are, you measure how many degrees you are north or south of the equator. Latitude reaches a maximum of 90 degrees at both north and south poles. Always label Latitude N, if north of the equator, or S, if south of the equator.

In chart navigation, use the scales on the right or left side to find your Latitude. These scales are broken down 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.

Navigating with Longitude

Return to your imaginary globe. To measure Longitude, you again divide the earth in half, but this time lengthwise. Locate Greenwich, England on your globe. Draw a line around the earth that intersects Greenwich and both north and south poles. Cartographers call this the Greenwich, or prime meridian – the birthplace of Longitude. Label the Greenwich meridian 0 degrees.

Longitude lines parallel the vertical Greenwich meridian to the east or to the west. To find longitude, you measure how many degrees you are east or west of the Greenwich meridian. Longitude reaches a maximum of 180 degrees on the other side of the earth, at the International dateline. You must label Longitude E, if east of Greenwich, or W, if west of Greenwich.

Use either the top or bottom of the chart to measure Longitude. Like Latitude, Longitude is broken down into degrees, minutes and tenths of minutes or degrees, minutes and seconds.

How to Convert Increments of Minutes

All nautical charts show minutes broken down into increments so that you can plot parts of a minute. For example, if your gps position shows 23-13N; 82-16W, there are no increments to worry about. But, if your gps position shows Latitude 23-13.278N; Longitude 82-16.786W, you have increments of minutes. Before you plot your position, round off increments to the closest tenth of a minute. Round off like this: Latitude 23-13.3N; Longitude 82-16.8W.

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

Some charts show degrees, minutes, and tenths of a minute. The minutes will be broken down 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 chart 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 earlier): Latitude 23-13.3N; Longitude 82-16.8W.
Multiply the increment of Latitude minutes like this.3 X 6 = 18 seconds.
Multiply the increment of Longitude minutes like this.8 X 6 = 48 seconds.
Plot: 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 Lat and Long onto the chart. Read the degrees and minutes from your GPS. Find the closest degrees and whole minute of latitude on your GPS.

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

Next, plot your Longitude. Use the same exact method to plot your Longitude. Make sure to use the top or bottom chart scales to plot your Longitude. When you've found your Longitude, place a pencil mark where the dividers touch the exact Longitude.

Find Your Exact Position

Align your parallel rules or any other straight edge so that the top long edge touches the Latitude pencil mark. Make the parallel rules or straight edge perpendicular so that when you draw in the Latitude line, it will be parallel to all other latitude lines. Draw a light pencil line across the body of the chart to a location close to where you marked the top or bottom Longitude scale.

Repeat this same method to align and draw in your Longitude line. Where the Longitude pencil line intersects the latitude pencil line shows your exact position. You should now have a cross. Place a dot where the two lines cross. Circle the dot. Erase the light lines drawn from the edges of the chart to clean up the chart and keep your plot neat.

Use these easy steps to find Latitude and Longitude on your nautical chart fast. With these sailing skills, you will be well on your way to become a confident sailing skipper-anywhere you choose to go sailing!

Source by John N. Jamieson

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