Dynamic Charts

A general refrain of old timers is why do the ENC look so different from the paper chart. The former is based on IHO’s S-52 colours and symbols, the other on INT1 Symbols & Abbreviations.

The S-52 has been developed keeping in mind that an ENC is displayed on a CRT or an LCD screen which is fundamentally different from a paper chart. Whereas a paper spread on a table reflects light the computer display emits light. Further the resolution of a typical monitor is insufficient for INT1 symbols to be seen optimally.

Yet if you notice S-52 closely resembles the INT1. So that the mariners get a comfortable feeling of dealing with familiar symbols.

Suppose if there was no compulsion to stick to existing symbols and the digital chart symbols were developed from the point of view of optimum exploitation of the new dynamic medium then, I dare say, the charts would appear radically different. Perhaps with drastic results till such time the mariners got used to it.

Let us take the example of the danger highlight symbol. It resembles a magenta-coloured screw-head and most mariners are stumped when they see it on the screen. Because of this isolated danger symbol we have already had the incident of an ECDIS fitted RO-RO vessel losing a propeller and suffering extensive underwater damages by running over a charted wreck which had being wire-swept to a depth of 1.8 meters. This was in January 2008.

The isolated danger symbol or the screw-head appears only if the user-defined setting of the safety contour is more than the depth over wreck. In this particular case of Pride of Canterbury running over the charted wreck of Mahratta the symbol of the wreck had changed its appearance due to the setting of the safety contour.

The ship was navigating off Dover which was familiar grounds for the Master. However, none from the bridge team consisting of the Master, Chief and 2/O realized the significance of the isolated danger symbol. Had they referred to the paper chart spread on the chart table they would had immediately recognized the wreck symbol.

So much for this dynamic symbol – that was actually designed to draw the attention of the OOW. Not only did it fail to attract the attention, it was simply ignored. For sure, the ECDIS has the facility of interrogation of a symbol when you move the cursor over it. Unfortunately in this particular case it was not used.

In my interaction with students preparing for competency exams, many of whom have undergone ECDIS course, I did not find any who understood the significance of the isolated danger symbol. As ECDIS/ECS usage becomes more widespread let’s hope the users become familiar with dynamic symbols and incidents like Canterbury rarely occurs.

On the other hand widespread usage of ECDIS will surely lead to development of more such uses, so that the digital chart becomes dynamic and changes its appearance due to external factors. Here are some examples which demonstrate how a digital chart is used differently from a paper chart.
- Guard Zone
A concept borrowed from the ARPA. Depending on the speed of a vessel the user defines the length and width of the guard zone which will extend as a probing beam from the ship’s head. Any shallow depths/contours or navigational dangers which happens to fall in this zone will trigger an alarm.
- Echo Referencing
This is an elegant way to accurately position own vessel. It requires positive identification of a stationary target such as a rock/beacon/islet on the radar. Then use the ARPA to acquire the target. The ARPA target should ideally coincide with the charted position of the rock. Many times it doesn’t because your positioning system is in error. The ECDIS allows you to lock this target on the charted object. The small difference in position becomes the correction which is applied on own position. The ECDIS uses the notation ‘R’ to indicate the referenced target.
- Weather Alarms
Set by the user depending on such parameters like heights, wind speed and barometric pressure. These are not chart symbols but an additional layer on the chart. It helps to indicate alarming situations in a particular region. Going a step further, you may have a voyage optimization software which will advise you on the course and speed of the vessel to avoid bad weather and choose the best passage to reach your destination.
- Ice Alarms
Depending on the ice information that has been received you may get alerts to indicate the presence of ice-bergs.
- Man Overboard Mark
The usual MOB mark tells you where the man fell. The ECDIS then generates another moving MOB symbol based on the calculated drift. Some software will also draw the Williamson turn for you to take the vessel around. It has received positive feedbacks from the end-user.

There was a time when a chart meant a wide sheet of paper spread on a flat table on which the outline plan of the coast, underwater details and navigational information was contained. In 2002 SOLAS changed the definition of a chart. The term nautical chart now includes a specially compiled database, for example an ENC, from which such a map can be displayed.

Today even this changed definition is outdated. The current definition implies static data. Whereas now in addition to the static hydrographic data the digital chart on the screen contains external data such as ice information, weather parameters and wave-heights. These data can be accurately collected only in real-time.

It's time we had a new definition of a chart which would reflect the dynamic character of a chart.