Chartplotter Protocols

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Chartplotters are powerful extensions of GPS technology and provide tremendous convenience, but they can get you into trouble if you’re not careful. While my focus is on vector-based plotters (digitized charts, which are the ones typically found on recreational sailboats), most of my observations also apply to raster-based (scanned charts) plotters.

Scale change matters. One of the nice things about using a chartplotter is that you can rapidly change scales by zooming in or out. However, you need to be careful when you do this because not everything will scale proportionally. A good example is cross-track error. When the scale is zoomed out, a boat will appear to be very close to the track line. But when you zoom in closer, it can become clear that, in fact, the boat is actually far away from the track line (Fig. 1). The reason for this optical illusion is that the size of the boat icon is constant, regardless of the display scale. In this case, the proper solution is to constantly monitor the numerical value of cross-track error.

Understand the small-screen effect. Although their sizes do vary, a typical government paper chart will be approximately 1500 square inches and commercial paper-chart books will be about 400 square inches. In contrast, a chartplotter that has a 7-inch diagonal screen will have a display of less than 24 square inches. One result of this size differential is that chartplotters have to make significant compromises when they display information. If, for example, all the information from a paper chart were to be packed onto a small screen, the display would become unreadable.

You could use the declutter or similar feature on the chartplotter to omit details, but this might remove potential navigational hazards. Or you could display just a small portion of the area, the big picture. More-sophisticated plotters have a split-screen function that lets you simultaneously view a zoomed-in image for details and a zoomed-out image for the big picture. But it still isn’t an ideal solution because instead of seeing one 24-square-inch display, you are now seeing two 12-square-inch displays.

The logical conclusion is that unless your chartplotter is very large—as large as the units aboard a commercial ship—it should not be the sole source for your navigational planning and tracking. Rather, you should use paper charts as well. Of course, a paper chart might not contain all the necessary detail either; small-scale charts, which cover larger areas, generally have less detail, while large-scale charts, which cover smaller areas, tend to have a lot more detail. Therefore, the preferred procedure when working with paper charts is to use small-scale charts to keep track of the big picture and large-scale charts for more detail and precise position-fixing.

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