GPS WAYPOINT MISTAKE
After a week cruising around the southern tip of New Caledonia, we are sailing back to Nouméa to provision, clear Customs and head for Australia. We had planned on a relaxing coastal sail, including a lunch and photo stop at the Amédée lighthouse that marks the entrance through the coral reef. Three weeks earlier we had sailed this way at the end of our passage from New Zealand, but at that time had been too tired to appreciate the lovely white lighthouse that graced the small islet just inside the reef entrance. Unfortunately our relaxing daysail has turned into a four-hour close reach into 30 knots as the leading edge of a strong high pressure system drives the barometer up rapidly. The short, steep seas building quickly inside the lagoon make for a wet and uncomfortable ride.
We reach the lighthouse, take a few quick pictures, and abandon the idea of lunch in the choppy anchorage. Instead, we turn toward Nouméa, sailing along the track we followed when we made landfall three weeks earlier. I ask Beth to put a route in the GPS and give me a course to steer. I give her the name of a stored waypoint I used to reach the entrance to Nouméa harbor when we made landfall. She drops below, finds the waypoint, and creates a route that starts from our current position and goes to the waypoint. She glances at the chart, notes the waypoint I had plotted when we arrived three weeks before, and eye-balls the route from our position to the waypoint. Satisfied that the course is clear of dangers, she reads out the bearing and distance to me.
I turn onto that bearing. A few minutes later the depth alarm goes off and our depth drops, reading 15 feet, 12, 8 . . .
As has been the case all week, the water is cloudy with sediment from strip-mining, the island's biggest industry, making it difficult to read water depth visually. We won't be able to pick out any water breaking on shoals because of the choppy seas and whitecaps from the windy conditions. By the slight change in water color we can see that it is shallow all around us, but we cannot see any deeper water that would lead us back to the main channel. There is an islet about a half-mile to starboard and a stake to port, the kind commonly used in the Pacific islands to indicate shoal water and not shown on the charts. Beth races below to check our GPS position on the French chart only to find that it shows us still in deep water. The depth is continuing to drop so I turn on the motor, stop the boat, hold it in position against the wind and call for Beth to tell me which way to turn.
There are too many reefs and small islets on the chart to identify where we are by fast eyeball navigation. The GPS position is also too far off to help. After some discussion we decide we should be okay if we motor very slowly and stay well away from the stakes. Our best guess is that we are to the south of the channel so we head north. After about a quarter mile of shallow water, giving a wide berth to a couple of stakes, we finally work our way back into the deeper channel.
Thoughts on avoiding the situation
It took some time to reconstruct exactly what happened and how we got so far off from where we thought we were. But to start with, we should have been following the basic rules of coastal navigation. Even if we didn't feel the need to take bearings on landmarks and plot our position on the chart, we should have been mentally checking off the various little reefs and islets we were passing and keeping track of our position. But we were lulled into a false sense of security by the GPS, something we have seen happen to dozens of other crews sailing in islands charted far less accurately than the US or Europe. We thought we were sailing down the exact same path we had taken three weeks before and just needed to pass from one waypoint to the next.
When we did sit down and sort out what had happened, it turned out that when we had come this way three weeks before I had noticed that the chart and GPS did not match and had corrected the position of the channel waypoints in the GPS. But I did not change the position of the waypoints I had marked and labeled on the chart. So, when Beth took our current position and checked for hazards along that course to the waypoint I had given her on the chart, she was not checking the same course the GPS had given her. The course and waypoint were just different enough to take us across an area of shoal water extending off of one of the small islets along our course. We were off the GPS course by less than a tenth of a mile, but in those coral-infested waters, that was enough.
If I had erased the old waypoint mark on the chart or used a new name for the corrected waypoint, Beth would not have been confused. Alternatively, she could have determined the chart and GPS waypoints didn't match if she had plotted it on the chart using calipers, but that would have taken several times longer and she did not think it necessary.
A chart plotter or PC charting system would have kept track of the waypoint change and immediately given Beth a correct course to check for hazards. At the time, we did not have either aboard. We now have both, and in this situation it probably would have kept us out of trouble. But in practice, we have found four problems with these systems.
First, the charts are hideously expensive, especially when, like most cruisers, we are usually just sailing through an area and use each chart only once. We often don't choose to buy the electronic charts for areas where we already have paper charts.
Second, I don't like having our laptop computer on/open when we're sailing, especially in vigorous 30 knot close reaches, as I am afraid the boat will take a lurch and something will get thrown at the screen or water will drip into the keyboard. These two points mean we still often plot our position on paper charts, so we're back to the situation in this case.
Third, the charts in many areas outside the developed world are not accurate and have not been corrected to GPS datums (as they were not in New Caledonia). As late as 2005 when we were in Stewart Island on the southern end of New Zealand our chart chip was out by a mile or more from the actual GPS positions.
Fourth, as mentioned in an earlier column, we have recently seen more "electronic chart assisted groundings" than any other type of accident. The charts look so real we start thinking they are reality and don't check them against any other navigational aid.
In the end, the GPS and chart plotters are just another aid to navigation, not the aid. The old warning about never relying on one aid to navigation is as true for GPS as for any other position-fixing device. We have to constantly remind ourselves to corroborate its readings with the radar, depth sounder, bearings on landmarks, or just eyeballing and identifying each passing island so we know where we are. These good habits die quickly when the GPS comes aboard, but preserving and encouraging them may well save your boat someday.
Thoughts on dealing with the situation better
We both panicked a bit, as the shallow water took us totally by surprise and we had become so reliant on the GPS we couldn't at first think of any other way to check our position. We should have stayed calm and composed as we always had the option of sailing out on a reciprocal course or dropping an anchor so we could sort out our position. We were in deep enough water and I could just make out the shallowest spots (by the slight change in water color) well enough to keep us clear. Once panic sets in for one person, it can make the whole crew lose their heads, and that's when the worst things happen. We have found that staying calm and clear-headed when we suddenly find ourselves in danger is often harder than in a storm or entering a strange harbor when we're expecting the worst.