The Offshore Voyaging Reference Site

Keeping Safe From Chart Inaccuracies

Here’s one simple tip that can save you a huge amount of grief…and possibly your boat:

Beware the dangers of assumed accuracy.

What I’m referring to here is the very human tendency—we all do it, including me—of assuming that because the presentation of navigation or weather information is really slick, that the underlying data must be accurate—the two are not correlated.


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Jo

Vector-charts don’t give us easy to understand feedback about limits of accuracy.

And sometimes feeling uncertain about where we are makes us safer, because we pay more attention.

Marc Dacey

That was, despite the translation, a good read. Thank you.

AlanS

Excellent article, another not quite as detailed, but also very good in similar vein by English writer Tom Cunniliffe
http://www.yachtingmonthly.com/sailing-skills/how-to-use-vector-charts-safely-33747
I certainly agree with his recommendation of running a raster chart in the background is vector is the primary system.
Personally, even with all the electronic information available I still like to have a paper chart beside me. I have the same (dinosaur?) preference when reviewing engineering drawings – CAD screens do not always portray the big picture.
I think the most basic rule for navigators and weather forecasters is to remember to ” look out the window” at regular intervals and not get mesmerized by glowing screens.

ChrisW

John, it’s not just cruisers and cruise lines, I personally know two military officers whose careers were truncated by chart accuracy — in one case the ship had to be broken up where she ran aground. I believe the charts in use were ENCs.

Richard s (s/v lakota)

am always happier with the current weather charts than a forecast (more like ‘nowcast’ usually), and those provided by bermuda wther svc via the net (weather.bm) are always beyond excellent and with nice range and detail…i have experienced more than a few mistakes verbalized from the noaa wthr radio broadcasts such as reversing tidal states as in refering to high when shhould have been low…major error never to be anticipated except by only the most paranoid…cheers, r in tampa bay for another month or so before heading to the antilles for a while

SidS

Your point regarding the position accuracy of floating aid to navigation (Bonus Tip #2) is well taken. In the 1970’s, I ran a search in Florida, between Rebecca Shoals and the Dry Tortugas, for a crashed Air Force fighter. Our search area was charted based on eye witness information from shrimpers who were anchored adjacent to a navigational buoy. After six unsuccessful days of searching, we discovered, quite by accident, that the Coast Guard had set the buoy 8 miles! from its charted position. Using the actual buoy position, we found the plane wreckage in a few hours.

Phil

John
I also use Nobeltec Trident and predominantly coastal cruise the Australia East Coast. Trident along with many other navigation programs allow for satellite overlay on the charts.
Do you have a comment as to the accuracy of using this to confirm chart accuracy.
I enjoy reading yours and other contributors articles.
PHIL
SV Kirralaa

Charles Robinson

Quick story, I was in the Greek islands and was heading to a very small harbour. I was chatting to another sailor and he said be careful of the reef immediately outside the entrance of the harbour. I checked the chart (paper) nothing, chartplotter nothing, harbour pilot, nothing. Tried google maps in satellite mode and bingo there is was, a patch of very light water. Now the dilemma, how was I going to avoid it if I didnt know where it was? So I took a view that if the satellite view was accurate on my tab to my GPS position before I set off, it would probably be ok 5 miles away. Anyway with stomach slightly in my mouth, I navigated in on google maps satellite – worked a treat! Still not convinced it is bomb-proof but ……..

Marc Dacey

A related tip in coastal areas is to use the “History” control to show all the available satellite images for the area under review. Aspects of tide and water clarity can reveal a great deal in certain images that are otherwise obscured in the default most recent view. I do not rely on Google Earth, but it is one of the more helpful nav aids in our route-planning process.