How to plot a position and course on a marine chart

Dear Capt. Paul:

This request is rather long, but it is the only way I can totally explain my needs. Recently, we were on a fishing trip about thirty miles offshore and we encountered a severe rain storm with lightning and thunder. This in itself was hairy enough, but to make matters worse, lightning struck a nearby oil platform and our radio and GPS units stopped working. By turning the units off and restarting they both came back on but the GPS unit had lost all of its route, tracks and waypoint memory as well as the internal maps that was on a memory card. All of my waypoints were now gone and I could not determine a course to reach our bayou’s jetties or even where they were located. The GPS was now displaying what I believed our current position expressed in a Latitude-Longitude format. I had a marine chart onboard and could see the jetties on the coastline but I did not know where we were located on the chart. I could not determine where we were on the chart in order to determine a course to reach the jetty. I knew we had to go north and finally we reached the shoreline but, we were miles away from the jetties. Can you describe how to determine a point on a standard marine chart.

— Harry S.

Capt. Paul’s response:

I bet you are you are glad you had the chart on board as a backup. It probably saved your trip and possibility your life. I hope you had all of your routes, tracks and waypoints backed up in a computer file. If so, it is an easy way to restore all of your waypoint data. Your predicament is why I enclose a copy of my Captain Paul’s Fishing Edge, with a recommendation that this “hard” copy be kept on your vessel as it has key position values shown and listed.

I strongly suggest that you take a course in safe boating which has a discussion on marine navigation. Such a course is usually free with you paying for the study materials. These courses are offered by the U.S. Coast Guard Auxiliary, the U.S. Power Squadron, as well as other law enforcement and private clubs. Most of these courses include a section on navigation which include plotting positions and setting courses via standard marine charts.

Many insurance companies offer a discount on their premiums if the client takes such a course. Either way, it is really worth your time and effort to take one of these courses.

The basic plotting tools are an official NOAA chart, a pair of dividers or drawing compass and a set of parallel rules to determine a course using the chart. It would greatly assist your efficiency if you were to practice the below listed suggestion before your next excursion.

As for the details in plotting a position, start with your chart laid out as flat as possible. If you have a stated latitude~longitude position or a location where you want to go to or to plot a course to make sure that it is in the same format as the chart which can be determined by noting the latitude~longitude marks shown along the borders of the chart. Most NOAA charts are shown in decimal minutes (DDD, How some of the larger scale, covering a very large area may not be in decimal minutes, but only full minutes. Remember when determining distances, one minute of Longitude equals one Nautical Mile.

Using the chart, you should find and note the bold numbers in the scales along the top and bottom and sides of the chart closet to your position degrees shown on your GPS unit or at your intended destination. You will note that there are heavy lines that form rectangles on the chart with the value of the position stated in the border scale. Be sure to also note the minutes shown on your position.

Select the nearest heavy horizontal and verticals lines from the needed position. Using the divider set and lock the divider at the distance from the position to the heavy chart line. Note that the indicated latitude and longitude numeric values in the border reference scale values.

Take a divider/compass, place one leg on the position and extend it to reach from the heavy chart lines to the position. One at a time, measure the distance. Noting the divider spread do not change it and move the divider to the scale on the nearest vertical border scale using it as a distance from the line. This will determine the latitude value. By using the same divider spread move the divider from the position-line to the border latitude scale. You may have to interoperated the distance from the major chart line to the indicated position on the scale. Remember that the latitude values increase moving higher on the map. If the divider is above the stated value line, you add to the determine number and if below the chart line you would subtract from the stated value. This value will be the latitude value of the position.

Now use the divider and measure the distance to the nearest vertical heavy line. This is to done to determine the longitude of the position in the same manner you used to determine the latitude, but now using the bottom or top scale values. Again, if the point is east or west of the main longitude heavy chart lines, you would add or subtract the position from the main heavy chart line.

If you know your position then plot the jetty entrance and manually enter it in your GPS, then select a GO TO function and you should be able to have the unit give a heading or course to the jetty. If the Go To function in the GPS unit is not working, but you have your current position, draw a line on the chart from the position to the jetty.

Place one of the Parallel Rule on the line. Select the nearest Compass Roses on the chart and shift or walk the movable rule in a series of steps so that it intersects the center of the compass rose on the chart. This intersection will give you an approximate course to the jetty.

Remember to back up your GPS to a computer file. It will save you countless hours of work if your GPS fails.

And, if possible bring a handheld GPS unit as a backup. Keeping this unit off and in a metal can, such as a military ammo can should protect it from a static electric discharge. Remember that the unit will require batteries.

— Capt. Paul

About Captain Paul Titus 192 Articles
Capt. Paul Titus has been responding to G.P.S questions on since 2000. He has been fishing and hunting in Louisiana since 1957. Titus holds a USCG license and conducts instruction courses in the use of GPS for private individuals and government agencies.