Hi Bruce,

Especially, the opening chapters on "The Limits of Accuracy"... I think one of the greatest dangers today, is that so many end users of electronic navigation have so little understanding of how these charts are created, and view them as being literally infallible. The risks of 'Over-zooming' are very real, and yet many sailors appear to be completely unaware of them...

Here's an excerpt that appeared a decade ago in OCEAN NAVIGATOR, but one really should read the fuller analysis in the book itself... The relevant portion re e-charts doesn't come until near the end of the article (Kudos to Doug R for being the first ever to link this one here, which I've done numerous times since) If any single book out there deserves a wider readership, I've always thought Nigel's should be the one...

http://www.oceannavigator.com/January-February-2003/How-accurate-are-our-charts/

In the end, the absolute accuracy of the average harbor, approach or coastal paper chart is generally not less than one millimeter with respect to the chart datum, which is to say that the charted positions of features should almost always be within one millimeter of where they would be if the chart was completely accurate. Put another way, most of the time the cumulative errors will not exceed one millimeter x chart scale. For a 1:40,000 chart, this is 40,000 mm = 40 meters, or 44 yards. There will, of course, be many specific bits of data that have been derived from older or less accurate surveys than the norm, and which, as a result, fall outside these parameters, sometimes by a wide margin, as will almost all charts of areas beyond the immediate coastal belt.

This is all a long-winded way of saying that the user of any chart should not be lulled into a false sense of security about its accuracy. Before GPS, there was always a degree of uncertainty about a boat's position. This led navigators to give a wide berth to hazards. In general, the techniques used to position hazards on a chart were more accurate than the nav tools available to the mariner.

Since GPS, this situation has been turned on its head. The equipment with which we navigate now has a positioning accuracy greater than that underlying the charts we use (including electronic charts, which are usually based on paper charts and old survey data).

best regards,

Jon