There were only two boats anchored at remote Swan Island in the Caribbean – the first two to visit that month, said the Honduran military guards there. The next leg of our journey south would have to skirt the poorly charted Vivorillo Cays and other outlying banks and reefs. The charts of the area were based on 19th century surveys, and we had heard reports of major discrepancies. Our companion boat got on the Northwest Caribbean SSB net and found another boat that had been there within the past few days, and soon they were exchanging waypoints over the airwaves. We used the waypoints and proceeded onward, happy to have another source of nav data, but wary as to how good they were.
Waypoint navigation is how most of us get about these days – plug the next destination into the GPS and away we go. It’s simple, accurate and generally safe. But it isn’t foolproof, and the evidence is awash on reefs all over the world. Possible sources of error include using an incorrect datum (not matching the chart datum) in your GPS or plotter, inaccurate charts and operator input error. The GPS system is accurate most of the time, but there are occasional glitches causing possible position errors. Combine all of these possible sources of error with an interfaced autopilot and an inattentive watch, who isn’t monitoring and using all available sources of navigation data, and I begin to wonder why we don’t sink our boats more often!
Plotting errors
I suspect a source of many errors is the practice of borrowing waypoints from other boaters. We all do it. Someone has been there ahead of us and it is easier, and often more accurate, to get a waypoint from them for the tricky entrance. GPS is very repeatable, and having a reading taken on location eliminates some possible sources of error. For example, the simple act of trying to determine the position of a chosen waypoint on a paper chart, possibly using a plotter, a pair of dividers and a pencil, introduces many possible measurement inaccuracies. Those who determine the positions of waypoints using an electronic chartplotter are not immune to error. Screens are often hard to read in bright light, the mouse may be balky and the scale might not be the best. Note how much the latitude/longitude changes with a tiny movement of the cursor. Is the program operating properly? Do you trust software to be right 100 percent of the time?
Of course, the best reason to borrow waypoints is the hope, often realized, that the original waypoint collector positioned his or her boat in the right place and hit the “save” button at the right moment. There are often hidden dangers not on the charts that can be avoided, or there might be a strong current flow that is wise to avoid, or possibly a string of lobster traps. In many cases these hopes are realized and we thank the absent waypoint taker as we safely arrive in harbor or avoid the unseen shoal.
Evaluate the source
But, hope is not a navigational tool and prudent navigators must use borrowed waypoints intelligently. The first safety move is to ascertain the origin of the point and its characteristics, if possible. For example, does it come from a trusted friend with plenty of navigational experience? Even better, did the waypoint lender recently use the same waypoint with success? Be wary of waypoints from second- or third-hand or unknown sources.
Generally, I find that most navigators leave their stand-alone GPS units set on the latest WGS84 chart datum. When borrowing waypoints you should ask, but I find that most people have no idea what datum was in use when the waypoint was taken. In many parts of the world this is not a big issue, but there are places where different datums can mean position differences measured in fractions of a mile or more. And, if a chartplotter was in use to take the waypoint, the plotter may have been set to a different datum in order to match the original paper chart’s datum. This can sometimes be avoided if your program or unit allows all positions to be read out using WGS84, even if the viewed chart was produced using another datum. There are also conversion programs that can translate positions from one datum to another. The bottom line is you should be comparing apples with apples and oranges with oranges. Try to use the same datum for your waypoints and charts. But when in doubt, assume the waypoint was taken using WGS84, and use the waypoint carefully.
Another thing to clear up with your waypoint lender is in what format he or she records the waypoints. Generally, everyone writes down latitudes first and longitudes second, and a lot of times it is abundantly clear which is which. But there may be times when your latitudes and longitudes will be very similar – be careful! A similar problem can happen when near the dateline or near 0° longitude – you might not be sure if the longitude is east or west of the line. It is good practice to always include an N or S, or an E or W, when writing down waypoints. When radioing waypoints say whether the latitude is north or south, or the longitude east or west.
I find that most navigators now record latitudes and longitudes using minutes and decimal minutes. The practice of recording seconds seems to have died with the arrival of all this digital equipment we use, but be sure you know in what units your borrowed waypoints are.
Plot the waypoints
OK, let’s assume we now have some borrowed waypoints from a trusted source. We know they’re WGS84 points, and we’re sure we’ve copied them correctly, using the proper units. The next step in validation is to plot the point out on a chart to be sure the waypoint represents the spot we want it to. This can easily be done in a chartplotter, or one can get out the tools and use a paper chart. Write the information down right on the chart or in the margin (one advantage of paper charts). Gross errors will immediately be apparent. Is the waypoint on land? Is it in the channel we wanted? Is the route to the waypoint clear of obstructions and in deep water?
However, in many cases, it will not be easy to plot the waypoint precisely enough to be sure it is a safe location. The thickness of a pencil line may actually obscure a bit of valuable information on the chart, like a sounding or a tiny rock. Watch out if your chartplotter automatically puts a large symbol on your chart based on a keyed-in lat/long entry – who knows what is hidden beneath?
Subtle position errors are not easy to spot. For example, in coral waters it would be impossible to chart every coral obstruction, and coral being a living (and dying) thing, the bottom changes all the time. What if our waypoint lender arrived on a cloudy day and just missed by three feet an unseen coral head? It is difficult to steer a boat to that type of accuracy, and we could very easily sail three feet in the wrong direction. Chances are also good that many coral harbors will have few, if any, other aids to navigation. For these reasons I do not rely on borrowed waypoints when making entry to harbors in coral waters – these places require good visibility and eyeball navigation to enter safely. The same problem arises in fogbound passages in Maine, where black rocks may be lurking just below the surface. Don’t rely solely on borrowed waypoints to enter dangerous areas in poor visibility, no matter how good the source!
On the opposite side of the coin, if the waypoint I am steering to is the entrance to a well-marked harbor I will use the waypoint with much greater confidence, having plotted it on the chart and confirmed my correct location by other means, including buoys, lights or radar. In fact, government and private sources, such as light lists and cruising guides, frequently supply published waypoints for many harbors and major aids to navigation. These can generally be relied upon, but I would still plot them out carefully and use with caution. As a former editor of Reed’s Nautical Almanacs I have personally alerted the Coast Guard to many published position errors.
Determine source of error
Sometimes we plot the borrowed waypoint and it appears to be in the wrong spot or even on land. Don’t immediately dismiss the waypoint as wrong. You may be plotting on a chart of a different datum, or the chart may simply be inaccurate. Charts based on old 19th century and earlier surveys tend to be less trustworthy on the longitude scale than on the latitude scale. You may want to use the latitude from your borrowed waypoint, but pick a longitude that is farther out from shore and more likely to be in deep water.
Some old pre-digital navigation techniques can be very useful when using borrowed waypoints. For example, let’s assume the waypoint indicates a good spot to turn and head toward the harbor entrance. The waypoint is in deep water, but close to a shoal on one side. When approaching such a waypoint, it is often prudent to leave the waypoint to one side, thereby keeping the boat in deeper and safer water and providing a margin for error. This can easily be done by watching your cross track error and maintaining a course to the safe side of a mark. Just be careful if you reset your “Go to Waypoint” partway along a particular track, as then the cross track error gets reset to the new course. In other words, your original track to the waypoint may be a safe one, but if you have reset your course part way along, be sure your new track to the waypoint is also safe.
Similarly, in conditions of poor visibility it is often important to choose a waypoint that is unlikely to also be the waypoint for fast commercial traffic or other craft. Major shipping channel entrance buoys can be a dangerous place in the fog as ships head to the same spot. In the case of ship channels there is often deep water just outside the markers. A waypoint on one side of the channel or the other is likely safer than one in the middle of the channel.
Increased alarm range
Consider increasing your alarm range when using any waypoints. My typical alarm range is one-tenth of a mile, but I might increase that to half a mile or more if I am uncertain of the positioning of a waypoint. This is also a useful technique when on long offshore runs to a waypoint. After several days or even weeks at sea, it is rather silly to steer to within a stone’s throw of a waypoint before taking other action to ascertain your position. Why not set a reminder five miles out that will hopefully wake up the watch enough to realize they are coming on soundings and should be extra careful about navigation? Set the alarm to go off before the boat could possibly approach any danger, or set the alarm at the maximum range of a lighthouse marking the approach. This is extra important if a plot of the waypoint indicates the charts of the area are inaccurate.
Just because someone gives you a waypoint doesn’t mean you have to use it. Instead of plugging in three places after the decimal, why not round off the waypoint to a more convenient number to enter that is also more likely to put the boat in safe water? Saved waypoints tend to be taken right on the edges of shoals and other obstructions. Why not choose a waypoint that is farther out and on the 20-meter sounding line? You can then confirm your position with both the depth sounder and the GPS. Or, you might want to move the waypoint to where you can also get a bearing on a lighthouse or a mountain to confirm your position. Use borrowed waypoints to increase your confidence in picking new, safer waypoints.
Compare with latest readings
Compare your borrowed waypoints to your own on-site readings at the earliest opportunity. For example, an ideal place to compare is at a sea buoy located in deep water. See if your own GPS reading agrees with the borrowed reading. That can give you confidence in other borrowed waypoints located in the vicinity, particularly ones located on the same chart. It is possible to enter offsets into many GPS units, but I am wary of the practice in case I should forget about the offset later. If you note a very small discrepancy between your waypoints and the borrowed ones, it may just be the result of slightly different boat positions or inaccuracies in the GPS system. If the differences become significant, look to the possibility of changing your GPS datum to match the chart datum, or you may be dealing with a chart of a different vintage than the original waypoint collector.
Sometimes it is handy to run two GPS units next to each other. I use a portable handheld unit for this purpose. One can be set to one datum and one to another, or one can be programmed with all the borrowed waypoints while the other provides a full-time display of your ship’s latitude and longitude. You can then save new waypoints in one GPS while still keeping the old, borrowed ones in the other. I find that I may get five or six waypoints for some harbor entrance, but I really only need one or two of my own for future use. The same thing can be done with a chartplotter and a stand-alone GPS unit. When making temporary changes in data or other critical settings, it is handy to put a prominent label on the GPS indicating this fact. I use a small piece of blue masking tape. Write the important notes on the tape and stick it to the GPS.
Borrowed waypoints can be another nav tool, but they should not be used alone.
John J. Kettlewell’s voyaging blog is at kettlewellcruising.blogspot.com.
