Sailing Navigation - How to Plot Lat . . .
Sailing Navigation - How to Plot Latitude and Longitude on a Nautical Chart
Any GPS receiver will find the latitude and longitude along your sailing routes at any moment. But as a skipper, have you plotted this information onto a navigational chart to check your position?
Fast Review to Get You Started
Cartographers (chart makers) create a grid-like web on your nautical chart. Latitude lines run in a horizontal direction. Longitude lines run in a vertical direction.
Navigating with Latitude.
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 (0°).
Latitude lines parallel the equator to the north or to the south. To find latitude, you measure how many degrees you are north or south of the equator. Latitude reaches a maximum of 90 degrees (90°) 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 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.
1° = 60'
1' = 60"
If your chart shows degrees, minutes and seconds, change the seconds into tenths of a minute. When navigating, it's much easier to work with degrees, minutes and tenths. To do this, divide the number of seconds by 6. Follow these two examples:
Convert 42° 25' 18" N
18" / 6 = 0.3'
Latitude: 42° 25.3' N
Convert 16° 09' 50" N
50" / 6 = 0.8' (rounded off).
Latitude: 16° 09.8' N
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 (0°).
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 (180°) 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. Use the same methods shown before to convert seconds to tenths of a minute.
Even with advanced electronics, all skippers must still be able to find latitude and longitude the traditional way. This will make your sailing navigation more accurate and you will always be able to see your vessels position on a nautical chart.
How to Plot Your Position With Super Accuracy
Imagine that you look at a chart without any land or water areas. All of the buoys, lights, towers, tanks, and chart symbols disappear. You are left with just a grid over your chart that has lines that run up and down and left to right.
Those that run left and right (or east and west) are lines or parallels of Latitude. Those that run up and down are lines or meridians of Longitude. These lines form a series of grids over your nautical chart.
At any moment in time, your sailboat will be located in one of those grid boxes--called a "position box". Your first step will be to locate your specific "position box". Follow these easy steps:
Tools You Will Need.
At 1030 you obtain the following position from your GPS: Latitude 41°17.0'N; Longitude
70°26.0'W. Follow the steps below to see how you would plot this 1030 GPS position.
Locate Your "Position Box" Before You Plot! (Illustration A above)
1. Write down your GPS Latitude and Longitude into your log.Label each coordinate N or S (for Latitude) and E or W (for Longitude). In the illustration above, you write down Latitude 41°17.0'N; Longitude
2. Concentrate on illustration A. Look at the right or left side of the chart--the Latitude scale.
3. Locate the approximate latitude 41°17.0'N. Notice that it lies between
41°10.0'N and 41°20.0'N. on the right side of illustration A. Place your finger at about this location on the right side of the chart.
4. Scan the top or bottom of your chart and find the approximate location of your Longitude 70°26.0'W. Notice that it lies between 70°20.0'W and 70°30.0'W on the bottom of illustration A. Place a finger from your other hand at this location.
5. Sweep your finger across the chart horizontally along your approximate latitude. Sweep your finger vertically along your approximate longitude.
6. Stop when both fingers intersect in the body of the chart. You are now inside your "position box".
7. Mark the corners of your box with a pencil as shown in illustration A. That way, if you need to leave the chart for any reason, you will be able to come right back to your position box to complete your plotting. Now go to the next set of steps below to complete the plotting of your position: 41°17.0'N;
Find Your Exact Position Fast! (Illustrations B, C, and D above)
1. Find the bottom edge of your "position box". Follow that Latitude line over to the right or left side of the chart.
2. Push the needle point of your compass in that line on the right or left side (latitude scale) of the chart. Notice in illustration B, you stick the needle point in the line marked 41°10.0'N. Next, spread the compass open until the pencil lead touches the exact latitude of your position. In illustration B, we spread the compass open until the pencil lead touches 41°17.0'N.
3. Lift the compass off the side of the chart. Take care not to change the spread of the compass. Move over to your "position box".
4. Stick the compass needle in the lower corner on one side of the "position box". Sweep the compass pencil lead over the vertical line of the box. Make a small arc (mark). Repeat this on the lower corner on the opposite side of the box. Use a straight edge to draw a line from arc to arc as shown in illustration B.
5. Find the right or left edge of your "position box". Follow that Longitude line up or down to the top or bottom (longitude scale) of your chart. In illustration C, we have followed the right edge down to the longitude scale.
6. Stick the need point of the compass in the longitude line you followed down--in this case 70°20.0'W. Next, spread the compass open until the pencil lead touches your exact longitude of 70°26.0'W.
7. Lift the needle point just off the longitude line and follow that same line back to your "position box". Stop when you get to the penciled line that you drew (step 4). Push the needle point of the compass into the chart where the penciled line starts. Sweep the compass lead over the penciled line. Mark your position as shown in illustration D. Erase all extra lines and marks to neaten the plot.
Practice sound sailing navigation techniques like these and you will become a more confident skipper, ready for whatever comes you way. You will have the peace-of-mind and know-how when the chips are down to find you way home--wherever in the world you choose to cruise!
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