How to Navigate the Intracoastal Waterway With a GPS

The Intracoastal Waterway consists of a series of protected rivers, estuaries and canals designed to support long-distance water trips from the Gulf of Mexico to New England, without risking a blue-water voyage in the Atlantic Ocean. Because the ICW is well-maintained, local nautical charts accurately cover most of the ICW's navigation hazards. However, the very dynamic nature of the ICW means that local conditions change seasonally -- more frequently, in fact, than commercial mapping software can often accommodate. In fact, publishers release annual routing books that include tide tables and changes to the placement of navigation aids.

Although most boats -- even smaller sailing vessels -- carry a GPS device of some sort, experienced mariners counsel against over-relying on a GPS and its on-board maps. In 2012, for example, the 12-meter yacht Okiana crashed on an obvious outcropping of rocks because the pilot navigated solely by GPS with no attention paid to sea charts or the horizon. No matter what navigation equipment a sailor uses, scans of the sea and good seamanship must always govern navigation decisions.

GPS devices are optimized for navigation over land, and may not work as well with underwater obstructions and rapid depth changes. Most smaller, hand-held units are accurate, but the maps they contain may not be current. GPS conflicts with local charts when the chart uses a datum inconsistent with the map on the GPS receiver. For example, many GPS devices use the WGS 84 datum -- a projection of the curvature of the earth for North America -- but national governments issue charts for their territories with a datum that's more accurate to their land. Thus, a GPS map using WGS 84 might be off quite a bit from the OSGB36 datum used in the United Kingdom: They're based on different geometric representations of the earth's curvature.

Because GPS devices are an imperfect solution for marine navigation, the best approach requires threefold attention to current local charts, the state of the water with its aids to navigation and some sort of geolocation tool like a GPS receiver. Use the GPS to provide a rough approximation of your location but rely on your chart and your binoculars to triangulate your precise position and remain alert for collision risks. Never rely on electronic navigation devices to control your vessel -- follow the U.S. Coast Guard requirement that a person always remain on watch on an underway vessel.