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Integrating GPS with traditional piloting

Jan 1, 2003

To the editor:

Nigel Calder's recent article on plotters ("Plot Twists," Issue No. 100, September/October 1999) presents an excellent comparison of the Yeoman GPS plotter for paper charts and a fully electronic chart plotter. He concludes with a personal preference for the Yeoman for its cost, for the scope and clarity of paper charts, and for the safety of plotting on paper should electronics fail. Most of us would agree that maintaining traditional skills is essential, and the Yeoman seems to encourage less dependence on digital wizardry and better maintenance of manual dexterity with the chart.

I would argue that, by using even less digital muscle and more manual skill, the coastal sailor can get the best of GPS while actually cultivating the traditional piloting skills that erode quickly when you simply follow a screen from waypoint to waypoint. After years of working up and down the coast of Maine with only a box compass and a chip log, I purchased a handheld Garmin 12 to gear up for an expedition along the coast of Nova Scotia in my Drascombe Longboat, Hummingbird. After some experimentation, I found a system in which my GPS supplements other piloting methods and avoids the most insidious aspects of digital dependence.

A well-rounded coastal sailor should have the ability to find his way safely without any electronics at all. The essential tools of the trade are compass, chip or taffrail log, plotting tools, and charts. The process involves tracking time, distance, speed, vessel heading, and bearings of other objects, and then representing this information on the chart so you know where you are and what is around you.

These tried-and-true concepts of coastal piloting are simple, but executing on the water is not. Laying courses, measuring speed, shooting and plotting bearings, extending the DR, adding in current vectorsall these are manual skills that can only be mastered and maintained by practice, particularly if you expect to perform well under pressure with cold hands in poor light on a lurching vessel.

Much of the lure of digital navigation is that it can relieve us of virtually all of the "drudgery" of manual piloting. Even on my bottom-of-the-line handheld unit, I can input a series of waypoints, link them into a route, and follow the screen to my destination. With that power in your hand, who is not tempted to put away the parallel rules and enjoy the ride? I have done it in dense fog in home waters, and it is great. But heaven help you if a thimbleful of salt water gets to your microchips, or a winch handle whacks your screen, and you have not a mark on a chart to show where you were last.

The system I use on Hummingbird keeps me honest by denying me any opportunity to simply follow the screen. Rather than using waypoints directly along my route, I enter two or three reference waypoints using the intersections of latitude and longitude lines on the chart. Setting out in restricted visibility, I plot a course; steer by compass; track speed, time, and current; and plot my DR as I would without GPS. At regular intervals, I use the go-to function to get a bearing and distance to the nearest reference waypoint. I plot this as I would any visual bearings I may get (with the added precision of distance off), update my DR, and continue it forward until the next fix.

The system has a number of advantages. It is cheapless than $200 for my unitwhich means you can buy two for about half the price of a Yeoman. Using the latitude and longitude intersections as reference waypoints is quick and simple and covers any route you may want to take on that section of chart. You can also use those reference points with the same geometric creativity you apply to visual marks, setting up warning bearings and turning points. Most important of all, it treats GPS as one additional source of data and brings it to the chart on an equal footing with your DR, visual fixes, depth, and other data. This encourages comparison and reconciliation of multiple data sources, which is the essence of prudent piloting.

GPS is a superb source of positional data. However, its supplemental functions tempt one to abandon traditional skills and rely exclusively on that one source of data. The ability to input waypoints and the effortless display of position, course to steer, speed, ETA, etc., are all impressive computational conveniences derived from GPS positional data. However, when you fully embrace these supplemental functions as your primary mode of navigation, you risk losing your ability to acquire, process and evaluate other sources of data and display them on the chart. If you treat GPS as simply one source of data to be integrated with other methods, the result is a more robust and enjoyable practice of the art of piloting.


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