Navigation involves knowing where your boat is, knowing how to reach a destination, and ensuring that the boat does reach that destination. Historically, navigation-especially out of sight of land-required many skills. Given a starting point from dead reckoning (calculations based on speed, time, and direction) or a sextant reading, you would track the boat's direction (with a compass) and speed (with a log) to continue to know the boat's position. Now, however, most mariners use a marine GPS chartplotter to navigate.
The historical difficulties of being precise in every aspect of navigation resulted in many shipwrecks. With the modern GPS system, in which a receiver on your boat interprets signals from Global Positioning System (GPS) satellites, you can know your exact location to within a few feet. A marine chartplotter is a piece of electronic equipment that shows, or plots, the boat's position on a nautical chart, allowing you to see where you are in reference to hazards, points of land, and everything else on the chart. A plotter also runs software that allows you to easily accomplish other navigational tasks.
GPS plotters include both dedicated marine equipment, called chartplotters, and computers running plotting software. Some handheld GPS units also are plotters, if they show the position of the boat on a chart. A GPS unit that provides the position only in longitude and latitude is not considered a plotter. For all types of plotters, the general principles are the same:
- The screen shows a nautical chart. The chart may be an exact representation of a NOAA paper chart (called a raster chart) or be a graphic representation in a format unique to the software (usually called a vector chart). More sophisticated plotter screens may show additional information such as photographic views, bottom contours, a radar overlay, and so on.
- The boat's position is shown in real time against the chart. The plotter may represent the boat with a simple dot, circle, or other symbol. Many plotters also indicate the direction the boat is moving with an arrow or other graphic indicator associated with the boat symbol. In some programs, the boat's speed may also be indicated graphically, such as by the length of the arrow. In one software program, for example, the length and direction of the arrow shows how far the boat will move in the next 10 minutes if conditions remain the same-giving an indication of how soon a turn may be necessary.
- Specific user-designated locations can be shown on the chart. Sailors enter these points in a variety of ways, such as entering the longitude and latitude or moving the cursor to a specific place on the chart and clicking. Special symbols then show these points, called waypoints, on the chart. For example, you might enter a waypoint for your destination, if you can make a straight-line approach to it, or you might enter a string of waypoints to follow if turns are required. Waypoints are now often printed on charts and cruising guides, and some software programs or built-in plotter programs also include useful standard waypoints. Waypoints are the basis of much of the functionality of a plotter.
Basic Plotter Functions
Originally, most plotters were fairly simple and straightforward in their functions for navigation. Like most software, however, these programs have become increasingly sophisticated and now offer more advanced functions than most of us will need. Described here are only the most basic functions found in almost all chartplotters and navigational software. For specific advanced functions, consult the user manual for a particular device.
- Go to. The go-to function is a simple way to navigate from where you are now to a waypoint destination that you can reach in a straight line. The screen tells you everything you need to know: what direction to go (bearing), the distance to go, the estimated time before you reach the waypoint (based on current speed), etc. Also provided is your heading, the direction you are actually moving in; if a current or the wind is carrying you off course (called cross-track error), you may not be moving in a straight line to your destination.
- Tracks. You usually have the option for the plotter to record your path through the water, leaving a "breadcrumb trail" on the chart. This can be useful, for example, if you are navigating through a channel or deep fog or in other circumstances when it is important to return by the exact path you took.
- Routes. A route is an organized set of waypoints. Typically you set up a route before leaving for your destination, planning the best way to transit a channel or get around islands or other features that prevent a direct go-to sail as the crow flies. Waypoints are collected into a route, and the plotter makes it easy to simply follow along from one point to the next. As with waypoints, the plotter keeps you informed of the distance to go and the estimated time remaining. Lengths of each leg and the entire route are also provided, along with bearings to steer during each leg. Depending on the hardware used, one might plan a complicated route while in port or at home, print out chart pages showing each leg and the vital navigation information, to be fully prepared. This is like having a computer do for you what pre-GPS navigators had to do on paper charts: draw in lines for course legs, using parallel rules to determine bearings from the compass rose and dividers to measure distances. Learn more about how to use chartplotter routes.
Again, these are only the most basic functions of chartplotters. With some advanced systems, the plotter can be integrated with a boat's autopilot so that steering is done automatically to follow the route. Other systems integrate information from other instruments, such as the boat's knotmeter and wind instruments, to make sophisticated navigational decisions about how to reach the destination most expeditiously.