Basic Navigationwith a Chartplotte
Navigation involves knowing where your boatis, knowing how to reach a destination, and ensuring that the boat does reachthat destination. Historically, navigation-especially out of sight ofland-required many skills. Given a starting point from dead reckoning
(calculationsbased on speed, time, and direction) or a sextant reading, you would track theboat's direction (with a compass) and speed (with a log) to continue to knowthe boat's position.
Now, however,most mariners use a marine GPS chartplotter to navigate.
The historicaldifficulties of being precise in every aspect of navigation resulted in manyshipwrecks. With the modern GPS system, in which a receiver on your boatinterprets signals from Global Positioning System
(GPS)satellites, you can know your exact location to within a few feet. A marinechartplotter is a piece of electronic equipment that shows, or plots, theboat's position on a nautical chart
, allowing youto see where you are in reference to hazards, points of land, and everythingelse on the chart. A plotter also runs software that allows you to easilyaccomplish other navigational tasks.
GPS plottersinclude both dedicated marine equipment, called chartplotters, and computersrunning plotting software. Some handheld GPS units also are plotters, if theyshow the position of the boat on a chart. A GPS unit that provides the positiononly 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 chartmay be an exact representation of a NOAA paper chart (called a raster chart
) or be agraphic representation in a format unique to the software (usually called avector chart). More sophisticated plotter screens may show additionalinformation such as photographic views, bottom contours, a radar overlay, andso on.
· The boat's position is shown in real time against thechart. Theplotter may represent the boat with a simple dot, circle, or other symbol. Manyplotters also indicate the direction the boat is moving with an arrow or othergraphic indicator associated with the boat symbol. In some programs, the boat'sspeed may also be indicated graphically, such as by the length of the arrow. Inone software program, for example, the length and direction of the arrow showshow far the boat will move in the next 10 minutes if conditions remain thesame-giving an indication of how soon a turn may be necessary.
· Specific user-designated locations can be shown on thechart. Sailorsenter these points in a variety of ways, such as entering the longitude andlatitude or moving the cursor to a specific place on the chart and clicking.Special symbols then show these points, called waypoints, on the chart. Forexample, you might enter a waypoint for your destination, if you can make astraight-line approach to it, or you might enter a string of waypoints tofollow if turns are required. Waypoints are now often printed on charts andcruising guides, and some software programs or built-in plotter programs alsoinclude useful standard waypoints. Waypoints are the basis of much of thefunctionality of a plotter.
Originally, most plotters were fairlysimple and straightforward in their functions for navigation. Like mostsoftware, however, these programs have become increasingly sophisticated andnow offer more advanced functions than most of us will need. Described here areonly the most basic functions found in almost all chartplotters and navigational software
. For specificadvanced functions, consult the user manual for a particular device.
· Go to. The go-to function is a simple way tonavigate from where you are now to a waypoint destination that you can reach ina straight line. The screen tells you everything you need to know: whatdirection to go (bearing), the distance to go, the estimated time before youreach the waypoint (based on current speed), etc. Also provided is yourheading, the direction you are actually moving in; if a current or the wind iscarrying you off course (called cross-track error), you may not be moving in a straightline to your destination.
· Tracks. You usually have the option for the plotter torecord your path through the water, leaving a "breadcrumb trail" onthe chart. This can be useful, for example, if you are navigating through achannel or deep fog or in other circumstances when it is important to return bythe exact path you took.
· Routes. A route is an organized set of waypoints. Typicallyyou set up a route before leaving for your destination, planning the best wayto transit a channel or get around islands or other features that prevent adirect go-to sail as the crow flies. Waypoints are collected into a route, andthe plotter makes it easy to simply follow along from one point to the next. Aswith waypoints, the plotter keeps you informed of the distance to go and theestimated time remaining. Lengths of each leg and the entire route are alsoprovided, along with bearings to steer during each leg. Depending on thehardware 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, tobe fully prepared. This is like having a computer do for you what pre-GPSnavigators had to do on paper charts: draw in lines for course legs, usingparallel rules to
Again, these are only the most basicfunctions of chartplotters. With some advanced systems, the plotter can beintegrated with a boat's autopilot so that steering is done automatically tofollow the route. Other systems integrate information from other instruments,such as the boat's knotmeter and wind instruments, to make sophisticatednavigational decisions about how to reach the destination most expeditiously.
Hardware and Software硬件和软件
A wide varietyof plotters and navigational systems are now available for all needs and allbudgets. Almost all of these fall into one of two general categories: dedicatedmarine plotters or computers running navigational software. Since each offerscertain benefits and has certain drawbacks, one should choose a system onlyafter carefully thinking through how it may be used for some time to come.
Marineplotters arededicated systems. Although some are handhelds that run on batteries and havesmall screens, most are installed on the boat and wired into the boat'selectrical system. Mounted systems generally have larger screens. All plottersare weatherproof and built to stand up to the harsh marine environment. Thenavigational software is built in, and often some or many charts as well,although specific additional charts usually must be purchased. Complete systemsrange from $400 for a handheld unit all the way up to $6000 for a full-featuredplotter with a 15-inch screen.
Advantages ofmarine plotters include their resistance to weather and ease of use withoutrequiring software installation or complicated chart downloads. Disadvantagesinclude higher costs (compared to an inexpensive laptop) and having to useproprietary charts purchased from the same company.
Computernavigational systems are essentially regular personal computers, usuallya laptop or PDA, to which a GPS receiver is connected.The computerruns navigational software and can have essentially all the same functions as adedicated plotter. Charts must be loaded into the computer. Except forexpensive "ruggedized" laptops, most computers must be shielded fromwater and moisture when on the boat. GPS receivers for laptops area availableas USB units or built into PC cards (under $100).
Advantages of computers include their lowercosts, their ability to serve other uses when cruising, the choice of variousnavigational software packages, and the ability to use them also at home toplan routes. Budget-minded sailors can use a free program called SeaClear
, which has all basic functions butfewer frills than commercial packages. SeaClear and some other software useelectronic versions of NOAA charts that are now free for download
on theweb. Disadvantages include having to protect the computer from the elements,having to install the software and set up the hardware connection between theGPS receiver and the computer, and having to install all charts.
Morerecently, smart phones
with anintegrated GPS receiver and running a navigational app can also function as achartplotter. Many inexpensive marine navigation apps
offerfunctionality that used to cost hundreds of dollars.
· Because a plotter can fail, always have paper charts onboard. Even the most rugged electronics are still somewhat delicate in themarine environment, and you don't want to end up drifting onto the rocksbecause of a sudden power failure. To be prepared for all eventualities,learn how to navigate with only yourdepthfinder and a chart
· Beware the temptation to stare too long at the plotter'sscreen, especially when mounted on the binnacle just above the wheel. Boatershave been known to suffer collisions with other craft (which of course don'tshow up on the plotter!) when they failed to keep an adequate lookout ahead.
· Watch out for cross-currents when headed for a waypoint -the boat can drift sideways and suddenly get into trouble if it leaves achannel or is swept off-course into a problem area.
· A plotter doesn't replace the need for good seamanship.If a waypoint is directly upwind, for example, you may have to use traditionalnavigation methods to choose which tack to take off the route-line to reachthat destination most effectively.