Inputting a Waypoint on a Plotter. To enter a waypoint using the plotter, open the waypoint menu, then move the cursor over the de- sired position on the chart for the waypoint and click on it. The plotter will automatically display the latitude and longitude numbers for the waypoint. This method avoids the potential human error problem of manually entering the lat/long numbers.


Inputting a Waypoint on a GPS from a Paper Chart. Use the chart to select the position of your desired waypoint, then determine its latitude and longitude using the method described earlier in “Plot- ting a Position on a Paper Chart” and input these numbers into the GPS. Once activated by pressing the GoTo button, the readout gives distance and bearing to the waypoint.


JUST HOW GOOD IS GPS


At its best, GPS is far more accurate than any ordinary boater needs. However, before risking one’s boat, one or two caveats must be consid- ered.


Chart Accuracy. In many countries charts are drawn with the utmost care, generally from meticulous survey data, but there are some areas in the world where this may not be the case. While GPS at sea can deliver a position good to 49 feet or less, the lines on a chart may be thicker than that, or the survey may have been conducted before GPS with a lesser degree of assumed accuracy. An electronic chart can often be zoomed in well beyond the intentions of the surveyor. Some plotters indicate when they’re over-zoomed – perhaps with dots on the chart. Many do not. The answer is always to leave a seamanlike margin for reasonable error, and give rocks a wide berth.


Satellite Coverage. Once a GPS receiver is well “warmed up,” satellite coverage is rarely an issue, but the instrument is only as good as the data its antenna is receiving. Check the satellite screen before you leave your slip, and take a look every so often to make sure all is well.


Datum. In the U.S. and many other areas of the world, GPS delivers its position using a lat/long datum called WGS84 (World Geodetic System 1984). However, a GPS may produce a position which is not consistent with the paper chart in use because the chart has been drawn to an older or different datum. It is possible to check a chart’s datum in the title notes, then enter the GPS menus and make sure the one selected coincides. If the GPS or the plotter doesn’t seem to coincide perfectly with reality, look for the buoys, use the depth sounder, and plenty of common sense.


This GPS satellite screen indicates signals are being received from 9 satellites and the accuracy of the GPS position is estimated as 10.6 feet. Four satellites are needed for a 3D fix (position and altitude). The position and number of satellites will vary as the earth rotates and the satellites change their positions in orbit. Satellite signals cannot penetrate solid objects. If used below deck in a boat, an external antenna may be needed.


The Sailing Environment Boat’s position


Waypoint


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