The Global Positioning System (GPS) satellite network has revolutionized ocean travel and maritime trade. It's also made recreational boating, both on lakes and on the open sea, much safer for boaters. It doesn't completely replace the skills needed to properly navigate on a course on the open water, but it significantly lowers the barrier to entry. If your primary exposure to GPS navigation is a turn-by-turn city navigation app, there are some differences to keep in mind.
Differences Between Marine and Land Units
Marine GPS units are superficially similar to some of the higher end automotive GPS units – they're usually touch screen devices, or devices with a few buttons on the side. They're waterproofed, and designed to be read outdoors in bright sunlight. They'll display distances in nautical miles (6000 feet units) rather than statute miles (5280 feet units), and they usually have tide information which will, when used for chart plotting, hide or reveal hazards based on depth. Marine GPS units can also integrate into your ship's radar, weather reporting and steering systems, and can effectively run the boat as a limited autopilot.
Because GPS coordinates are accurate down to only 10 yards or so, it is possible for a boat to be longer than the probable error on a GPS's display on the map. This is one of the reasons why they're mounted on the mast – even if the display is repeated in the steering cabin. If your GPS is mounted at the back of a 40-foot boat, and your probable error is 30 feet, you can run the bow into a hazard that the GPS won't warn you about. You should also make sure that the placement of your GPS system doesn't put it under a metal roof or metal spar work, as those can obscure sightlines to the satellites overhead.
Steering With A GPS Chartplotter
If you've used a turn-by-turn navigator on a land-based GPS unit, you'll understand the basic concept of how a GPS chartplotter works. Your GPS coordinates will be overlaid on a navigation chart of a body of water, and you'll be able to plan out your trip, and even program your waypoints, before you leave the marina. Where GPS is different from the land-based version is its integration with other equipment on the boat. Unlike most cars, a marine GPS can be tied into the controls for steering and will hold a steady course for you on a given bearing, or when following a pre-programmed course. It can also integrate with your boat's radar set, and show you radar contacts on the chart automatically. Several GPSs, like their automotive counterparts, come with voice modules, to make the experience more interactive. Many maritime GPS units can also overlay weather information pulled from your ship's weather tracking radio.
Navigating With GPS Assistance
GPS navigation still requires knowledge of compass bearings and how to correct for compass declination on the deck, although a GPS receiver makes this much simpler than it used to be. There's a very good chance that your GPS unit can give you the proper declination correction factor for your position, based on your coordinates. A good GPS system is still hampered by the quality of your charts. The greatest risk is missing underwater hazards, or buoys that aren't marked – trusting to the GPS and not using your eyes or taking an active role for spotting, can still lead to a grounding.
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