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Author Topic:   Digital Charts and Zoom Levels
jimh posted 12-15-2014 12:41 PM ET (US)   Profile for jimh   Send Email to jimh  
At the end of November 2104 a yacht participating in the VOLVO OCEAN RACE, TEAM VESTAS WIND, a 65-foot sailboat, ran onto the outer reef of Saint Brandon, also known as the Cargados Carojas Shoals, while under full sail and making about 17-knots or more. The crew survived the incident without any significant injury, but the custom racing sailboat lost all her rudders, dagger boards, and most of the hull bottom in the stern of the boat. Amazingly, the rig stayed intact.

For a boat equipped with the best and most modern navigation equipment available--GNSS receivers, excellent digital charts, and advanced chart plotting gear--the failure to take notice of the atoll, its reef, and its islands has been a topic of conversation among racers, sailors, and navigators. One explanation offered has been the problem of ZOOM LEVEL.

On digital charts represented on electronic chart plotters, the details of the chart that can be seen clearly when zoomed into a large-scale presentation are often omitted and lost when the same chart is viewed zoomed out to a small scale.

I discovered this problem with my own charts a few years ago, when we ventured for the first time into the remote and sparsely charted area of the North Channel known as McGregor Bay. As we crossed the main part of the bay, my chart plotter (at that time a rather simple, small, and black-and-white-only Standard Horizon model) and C-MAP charts showed a good view of the bay, its water depths, and many rocks and island. But when we began to gunkhole northward, threading among the many small islands, the chart display became very coarse, and showed only jagged vector blobs for island, no depths, and no details.

On that particular passage we were following another boat, Dave on GAMBLER, who had a much more modern, much larger, and in-color GARMIN chart plotter. Ironically, his Garmin charts, which had showed the area in great detail and allowed Dave to pre-plan on his computer and import a very detailed route through all the hazards, were mysteriously not working on the boat's chart plotter. (This was not a problem of zoom level but some odd problem with chart authentication or licensing, as I recall.) He was just carefully following his pre-planned route, and we all kept an eye on our depth sounders and looked out for rocks.

We made it through a zillion hazards, arrived at Rat Portage, our anchorage destination, and settled in for a delightful day and night at anchor. On the next day, as we were exiting, I happened to hit the ZOOM IN button on my chart plotter. I discovered--much to my surprise--that my C-MAP charts now showed McGregor bay in exquisite detail. Every rock, every shoal, every twist and turn was beautifully draw on my chart plotter, where before had been only the most vague block outlines of just the larger islands and no depths or aids to navigation. It was at that moment that I discovered the problem of Zoom Level.

It may have been Zoom Level that mislead the crew of TEAM VESTAS WIND into neglecting to go around a very well known hazard that was on their electronic charts but perhaps not being displayed to them because of the zoom level at which they were viewing.

For more about what happened to TEAM VESTAS WIND in their navigation, I recommend this excellent article:

jimh posted 12-15-2014 01:19 PM ET (US)     Profile for jimh  Send Email to jimh     
The zoom level problem is one of the presentation of the chart data on vector charts. The chart publisher has included all the data but the chart plotter has to organize it for presentation to the navigator. Exactly what zoom level will show what layers of data is probably up to the chart plotter maker.

In a related article, I discovered that selection of CHART OPTIONS could result in some very dangerous omission of hazards. For some interesting discussion of chart accuracy and layers of data presented, see

Electronic Charts for Isle Royale

Don SSDD posted 12-15-2014 08:45 PM ET (US)     Profile for Don SSDD    
Jim, I have 2 neighbors in an ocean race, they finished it today from the Canarys to Granada. The skipper, Derek Hatfield, has sailed solo around the world in a race. He pitch poled off the Cape of Good Hope in one race, he had about 7 different satellite tracking systems at the time.

Don't think he ran into any reefs lately though. He parks the boat on a mooring in front of my house.


jimh posted 12-17-2014 12:30 AM ET (US)     Profile for jimh  Send Email to jimh     
Don--My neighbor is an attorney that practices criminal law. He seems to be very much like your neighbor: he doesn't have much to say about the problem of navigating with electronic charts and Zoom Levels.

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