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How to Read a Nautical Chart by Nigel Calder - Book Review

It's much more than what its title suggests.


How to Read a Nautical Chart by Nigel Calder

How to Read a Nautical Chart by Nigel Calder

The title of this book of so much value for most sailors doesn't convey its real importance, since most boaters seem to feel they already know how to read a chart well enough to stay safe on the water. The subtitle takes us a step closer: "A Complete Guide to Understanding and Using Electronic and Paper Charts." What neither title does, however, is convince the casual browser of how critical the issues covered in the book can really be. While it may seem grandiose, I'd rather see stamped in big red type on the cover an admonition something like "Read this book or risk sinking your boat and perhaps dying as a result of chartplotter inaccuracies!"

For every boater who ever enters unfamiliar waters - or even strays a few meters into less familiar waters in their home sailing grounds - parts of this book should be considered required reading. Myself, I've been using paper charts for three decades and electronic devices on a wide range of devices since they first came available, and I learned some scary information here I'd had no previous understanding of and will have a much more cautious approach to interpreting charts (and my position on them) henceforward.

Nigel Calder
How to Read a Nautical Chart, second edition
International Marine, 2012
271 pages, soft cover

What It's About?

To begin with, "How to Read a Nautical Chart" is essentially a comprehensive explanation about almost everything related to marine charts: all the basics, the historical underpinnings, new developments, and a complete, highly detailed guide to all chart symbology. All this makes for great reading for mariners interested in charts, though at first glance many chapters may seem dry reading for recreational boaters who just want to "get there."

Particularly important for those who are not professional mariners, however, are the chapters on chart accuracy, which I'll focus on in this review. I'd speculate that a likely sizable majority of recreational sailors, fishermen, and casual boaters these days are using electronic charts, whether on a dedicated chartplotter on their boat or in an app on their smart phone or other device. More than a dozen inexpensive charting apps are now available for Apple and Android devices, and an entire generation of boaters no longer learn traditional seamanship and navigation when it comes to knowing where you are on the water: they just look at the small screen and see their boat positioned on the chart, and as long as they keep that little boat symbol away from hazard symbols on the screen, they should be fine, right?

Why is it then that boats keep hitting underwater rocks and going aground and sometimes experiencing sinking disasters? A lot of that has to do with how well boaters understand the accuracy of the chart as it is being used, at whatever zoom, on their device. This is the great strength of Calder's book for every boater: a comprehension of the accuracy of the chart and plotter and how to safely interpret one's position relative to obstacles.

Why It's an Important Book

The bottom line is that any boater needs to pay great attention to issues of chart accuracy in order to avoid nearby obstacles. Calder explains in great detail the possible range of error on most charts at different scales, with particular emphasis on modern electronic charts. The discussion of pixels and different screen resolutions may be more detailed than most boaters need, but it establishes the basis for his sometimes shocking conclusions. Only 60% of US charts, for example, meet stringent standards for accuracy. Even with the most modern updated chart data, the shown location of an underwater obstruction can be up to 30 meters, or about a 100 feet, from where you think it is - due to a set of factors fully explained in the book but too complex to summarize here. That fact alone should stand as a warning to never cut it close when in a problem area.

But the conclusion is not even that simple, for in many narrow channels or other tight areas you do need to trust your plotter for safe passage - but you also need to understand the range of error. Remember too that GPS localization is generally considered accurate to only about 5 meters. Depending on the device you're using and its resolution compared to the chart's accuracy at its scale, it is easy to err also by over-zooming in on a raster chart's detail or not setting a vector chart's visualization to the appropriate scale. Again, overdependence on what you think your screen is showing can rapidly lead to trouble.

Overall, the one downside of this book is its enormous detail of explanation, which can rapidly become overwhelming if you're looking for a quick conclusion. For example, there is no simple answer to the question of how close you can safely come to a rock shown on your chart. The answer depends on the chart's data, the date and thoroughness of soundings, the people who did the surveying and the chart preparation, the people who translated the paper chart into an electronic chart, the resolution of your device and how it modifies pixels when zooming in and out - and still more factors.

The thing one should remember, after reading Calder's exhaustive discussion of all these factors and perhaps years later forgetting most of the detail, is that you should always err on the side of caution and use all your best navigation and seamanship skills. Don't just glance at your iPhone and guess how close you can cut a corner!

In the end, there's still as much art to navigation as science, even though our electronics seem to tell us otherwise. And anyone who browses this book will become a safer mariner as a result.

Disclosure: A review copy was provided by the publisher. For more information, please see our Ethics Policy.

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