Some of the greatest technologies ever developed became obsolete in our lifetime: moveable type, for example, and telephone wires and film photography. Foremost among these is celestial navigation, an arcane science that once took decades to master. Today, anyone can navigate faster and better than the most skilled seamen of yore with the help of an inexpensive and readily available global positioning system (GPS) unit.
Mind you, contrary opinions still abound. Yes, the government can degrade the GPS system at will or even shut it off. Yes, the satellites might be shot down. And yes, it leaves you just one faulty transistor away from disaster. That said, prudent seamanship demands use of GPS as the primary means of navigation, nowadays, and no boat should be without it. For backup, use a second (and third or even fourth) GPS device or else use a sextant, your choice. If you choose a sextant, you will also need a chronometer, and good luck in high seas, storm, overcast skies, or fog, which in many areas is most of the time.
Navigation consists of two things: knowing precisely where you are and knowing in which direction you should be heading. Any GPS unit provides a continuous fix that tells where you are, instantly and always, with precision that Bowditch could only dream of. Most will also plot your course and speed. It is still important to know the basics of traditional navigation, primarily for the concepts and vocabulary, so that you can read about it and discuss it with others. A good way to gain this knowledge is by taking the navigation course offered evenings or weekends by your local USCG auxiliary.
GPS unit
charts
Many GPS units contain charts that are condensed and omit some of the data found on a printed chart. They might lack on-shore detail, for example, or bottom contours. Such units can be used for steering, but not for navigation, and should be used only with a full and complete chart at hand. Be even less trusting of GPS units that have not been updated with the latest charts, as these are often not merely incomplete but downright wrong.GPS
error
GPS units occasionally show erroneous data. A good GPS device of recent manufacture is accurate to within 9 feet only 95% of the time. The errors can be caused by the satellites, atmospheric conditions, or most commonly by the device itself. The way to deal with such errors is by having two (or more) GPS devices and comparing their outputs. If they agree, and particularly if they are acquiring more than a few satellites, you can have high confidence in the fix.Electronic charts in GPS devices are just as reliable as paper charts of the same vintage and equally susceptible to the same shortfalls as a paper chart. The important thing is to keep them up to date, including any pertinent notices to mariners. Expect the data to have errors occasionally. Always plot a course that has a margin of safety to allow for such errors. Stay alert to recognize errors when they occur.
Date of
last entry
Perhaps the single most important piece of information on a chart will be its date of issue. It is dangerous to use an out-of-date chart and foolish to purchase one. NOAA publishes on their Web site an online list of every current chart, identified by number, along with its printing date and date of last entry (DOLE).On a chart plotter, you can pull the chip and check the date printed on it or else look for an Info or About choice on the main menu. On a computer or handheld device running navigation software, look for an italic “I” icon in the upper or lower right corner of the main screen. All US charts with the same DOLE will have identical features, unless some data has been abridged, because they all derive from the same government agency.