New Island or Old Chart?

23 April 2009 By Kate Hubert

If you were cruising in the ocean off Tonga in March, you might have been forgiven for thinking something was seriously wrong with your chart plotter. Thirty-nine miles northwest of the Tongan capital, Nuku'alofa, the chart shows just a few tiny islets – but your eyes would have told you that a much bigger island was right in front of you....

Okay, so it’s pretty rare for submerged volcanoes to erupt and form new islands – we can’t expect the cartographers to keep up that quickly! But do you trust your electronic charts? And does your crew really know how to use them?

As the digital age swallows up the old paper charts and spews out pretty pictures with itty-bitty ships on them, we’ve come a long way from areas marked “Here Be Dragons” – but should we still beware?

According to SOLAS, any ship over 500gt or carrying more than 12 passengers on an international voyage must carry “adequate and up-to-date charts.” Even for yachts under 500gt, SOLAS is considered best practice by many in the yachting industry. So what charts meet those qualifications?

Most yachts now use digital charts and plotters, but they must be IMO-approved Electronic Chart Display and Information System (ECDIS) systems in order to comply with SOLAS. ECDIS systems use charts that have been digitized from approved Admiralty and Hydrographic Office sources. A back-up also is required onboard – usually a second, stand-alone ECDIS and GPS in case of electrical failure, or sometimes paper charts.

Today, vector digital charts or Electronic Navigational Charts (ENCs) are favoured over for use with ECDIS over raster charts. ENCs allow layers of detail to be added to or removed from the chart as the operator requires, and can be set to give alarms, e.g. clearing lines and depth contours. Raster charts, on the other hand, are simply scans or copies of the original charts, so data layers cannot be added. (Although it seems as if vector has won the battle, raster does still have its proponents, who point out that vector information is not “better” because it looks good and is easily manipulated, since the layers have still been generated by simply tracing over and copying the original raster/paper data.)

Like any technology, however, an ECDIS is only as good as the information it’s fed. Although the image you see on screen is modern and digitised – it may well be based on surveys undertaken over 100 years ago using sextants, paper and pencil to work out the calculations.

There’s a concern that the attitude, “because it’s on the screen it must be true”, can creep in when using electronic chart plotters. As one captain said, “I was sailing into a strait between two large islands; the chart plotter showed a sizeable islet to one side of the channel so I set my course to steer round it. However, as I turned into the channel I could see no islet. Even though it clearly wasn’t there, I still decided to steer well clear and monitor the depth. Radar overlay on the chartplotter confirmed it – there was no islet. When we checked the paper chart it simply wasn’t there....”

A computer is not an intelligent cartographer; it’s always a good idea to use your own common sense – and eyes – in unknown waters, as well as additional help from radar overlay, pilot books etc. A chart plotter is not a computer game; it’s easy to fixate on the brightly coloured screen rather than take a good look outside!

Proper training for the crew also is important. The inexperienced user often zooms in way too far, losing information about depths and dangers ahead. In one highly embarrassing shipping accident, a tanker rear-ended the ship in front of it because the tanker’s crew was so over-zoomed that the channel, radar blip and AIS signal of the boat in front did not appear on the screen.

As Simon Slater from marine navigation specialist Chersoft says: “Good cartography has been sacrificed in the temple of zoom.”

Do you trust your electronic charts? Have you ever encountered any serious charting errors?