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March 2011 Issue

The Limitations of Electronic Charting
By Alain Victor
Technology continues to gallop ahead, but the reins that could provide control have yet to be invented. This problem is particularly true in the electronic navigation field, where the global positioning system (GPS), the electronic chart system (ECS), the electronic chart display and information system (ECDIS), and radars are continually being renewed, updated, improved and interconnected.

These new navigation systems offer undeniable benefits to the savvy user. But there are concerns about whether the majority of us—the normal human beings—can keep up with these ever-changing navigational instruments. These questions become increasingly relevant when automatic identification system (AIS) information is being used to prevent maritime collisions.

As a navigation simulator trainer for half of my maritime career, I have seen many interpretation errors when faulty information is being displayed. The trainees would see that something is not correct, but the cause of and solutions to these errors do not always come easily.

Let us look at an example of a common situation that would not cause a problem in good visibility, but will create great difficulty if Mother Nature decides to blanket our horizon with a thick coat of fog. Assume it is the start of winter with calm winds and a current in the opposite direction of the ship's course-over-ground (COG). Vessel speed is eight knots and the heading is 120° approaching a narrow stretch of water on the right side of the channel, as indicated by the electronic chart and confirmed by beam radar ranges (the buoys have been removed just prior to winter icing).

In this scenario, the AIS signal is displayed on the radar screen (at the Maritime Simulation and Resource Center, where I provided training before retiring, the integrated bridge allows the trainees to either put the radar overlay on the ECDIS or vice versa). At this point, the radar detects a target that is displayed on the heading marker and is approaching at a relative speed of 15 knots. The AIS icon is displayed 5° to the right of the heading marker (as on the ECDIS). The ECDIS's COG vector points to the AIS icon.

Because of the reduced visibility, the first obvious question in the observer's mind will be: 'If there were better visibility, where would I see the other ship?' Is the vessel dead ahead, as the radar target shows, or is the vessel 5° to starboard, as indicated by the AIS icon? Or is the oncoming ship somewhere else entirely because of other errors?

If you have never thought of such a situation in clear weather, you will very likely struggle in situations with low visibility, especially if doubt hampers your judgment (and it will, as doubt could be said to be, in some cases, the first step toward safety).

Here is what actually could have happened in our example situation, along with an explanation of a few other error possibilities. In our scenario, a small gyro drift has created a temporary error of 5° east (low). The gyro says we are steering 120°, but we are on a true course of 125°. The radar echo is displayed on the heading marker, and this is correct (the oncoming vessel is dead ahead), but it is 5° off with respect to true north. In this case, the AIS gives the correct geographical position and puts the vessel on the correct place on the chart. The COG shows what we are really doing with respect to true north.

The paradox is this: The radar is telling us that the ship is dead ahead—and this is true—while the AIS is telling us that the ship is 5° on the right of the heading marker, which is also true considering that each has a different reference as north (the true north and the gyro north). The COG shows that we are on a collision course with the AIS icon and this also is true.

You may ask yourself, why is the heading marker aligned on the gyro instead of the COG? Because for anti-collision purposes, it is the relative position of targets that must be displayed with regard to the ship's fore and aft line (the heading marker).

Imagine now that you also have an unknown heading marker error in the situation depicted here. This could be very confusing as we really cannot be sure at first of the source of the problem. How can one tell, as the ship is moving ahead toward a difficult passage, which system is the culprit?

No single system is foolproof. There are times when AIS indicates an erroneous position for another vessel. For example, I have in my files a 'recorded accident' that did not happen. A vessel's AIS displayed on the ECDIS of another vessel left the main channel in piloted waters and hit a bridge, only to come back on track after the 'virtual accident.' To this day, what caused this error is still unknown. There is also the possibility that our equipment has not been properly calibrated or programmed or that its errors have not been identified.

All of these problems bring us to the downside of having very advanced technological electronic instruments: an overreliance on the data. Too many are using their ECDIS or ECS as the main navigation tool and often reject information that does not coincide with what is displayed on their screen. There are many well-documented incidents that substantiate that statement.

Here is my answer on whether AIS should be used as an anti-collision device: AIS does provide very valuable information that can be used to anticipate a situation, but the icon should never be used as a real 'target.' As we have seen, it could well be displayed in the wrong direction with regards to the ship's head.

New technology can sometimes catch you off guard, especially in restricted waters and when everything happens fast. If you want peace of mind and safe passages, prepare before arriving in any situation that could result in an incident due to a lack of correct planning.

Alain Victor began his ocean career in 1959 in the French archipelago of Saint-Pierre et Miquelon off Newfoundland. After a few years as a fisherman in the North Atlantic, he moved to Halifax, Canada, learned English, sailed on tankers and worked up the ladder to Master Foreign Going. He has been a navigation simulator instructor since the early 1980s.


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