NavList:

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From: Bob Goethe <NoReply_Goethe@fer3.com>
To: luabel@ymail.com
Sent: Thursday, March 5, 2015 11:36 AM
Subject: [NavList] Re: Modern celestial navigation: when and why?

>>I feel compelled to ask for specific examples of where the "accuracy" of GPS has caused ship's groundings.<<

Lu, I just joined the list and have been enjoying perusing some of the archives.  I ran across your question, and while I don't know of any ship groundings as a result of GPS, I have some thoughts from my own experience as to how this might occur.

In January, my wife and I were sailing aboard a 37 foot boat in the British Virgin Islands, and we discovered that our GPS position was 45 or 50 yards further south, and slightly east, than the charts would indicate.  Now, I am quite sure that our GPS latitude and longitude was correct.  The issue is that the nautical charts for the BVI are offset slightly. 

The charts have all the islands and other features in their correct positions relative to each other, but the whole island group is set 45 or 50 yards too far north.  Now, this is not a problem for you if you are using the conventional tools and techniques of coastal navigation:  hand bearing compass, paper charts, drafting compass and parallel rulers.  You look at this smokestack and that point of land, take bearings with your hand compass, and know you are here.  And at that moment, you don't care where here is in terms of global coordinates.  You care about where it is relative to the well-charted, submerged rock that is 800 yards offshore of a particular island.

When it comes to missing that submerged rock, if you are depending on your GPS unit (which tells you - correctly - that its position is accurate to within, say, 2.5 feet) and you try to shave that rock too close, you could run aground.

This failure of charts to register properly with GPS is pretty well known among sailors in western Canada.  It is not uncommon to be tied up to a dock somewhere, and have your GPS indicate that your boat is 30 yards inland.  There is one slightly famous little island up in that region where Vancouver Island almost-but-not-quite touches the mainland where your GPS will show you on one side of the island when you are in fact on the other.  You simply MUST use traditional coastal navigation techniques in that part of the world:  the charts are drawn as carefully as possible...but the registration with the global coordinate system is simply OFF.

What matters in coastal navigation is not, speaking precisely, absolute position, but relative position.

I tried to research how our Canadian cartography could be so far out of whack, relative to GPS, and the best I could figure is that we reference all our map distances to someplace called Meades Ranch, in Kansas.  And if you make little tiny surveying errors here and there, by the time you get to northern British Columbia, you end up putting an island in the wrong place.

There is another factor here, which is human.  I am ambivalent myself at having a chartplotter in the cockpit where I can watch it from the wheel, as opposed to having it down at the nav station in the cabin.  At one level, I like having my little boat icon moving across a chart on the screen before me as I steer.  But I am pretty sure that to the extent my attention is focused inside the cockpit, my situational awareness of what is outside the boat has degraded.  This is not a GPS accuracy issue, per se, but is a safety-at-sea-as-pertains-to-GPS issue.

In the BVI, the islands were last charted over 100 years ago..

     (yes...I know the British are supposed to have a reputation as a seafaring people,
      but when it comes to good charts, you are vastly better off in American or Canadian 
      waters, where the governments take their commitment to safe navigation much more 
      seriously; the British seem to think that if they know where the Greenwich meridian is, 
      they have done their whole duty to the world)

...and being an area where there is coral - and coral both grows and dies - even if the depth soundings that you see on your GPS chartplotter were accurate in 1885, there is no guarantee that they are accurate today.

Water in the Caribbean changes color with depth.  So as you are coming into a bay or harbor, you want to be watching the water color like a hawk.  Lose situational awareness by tracking with the GPS too closely while you are at the wheel, and you might find your vessel in water that was plenty deep enough for you in 1885...but shallow enough to strike bottom today.  Better to keep your focus outside the cockpit.  If you have a crew, assign one to be using the handbearing compass watching for danger bearings, one to stand on the bow watching the water change color, and one in the cabin (or even in the cockpit beside you) watching the GPS.

Every so often a vessel runs aground (like that Greek container ship that went aground on the Astrolabe Reef in New Zealand in 2011), and you ask "What kind of idiot runs aground on a well-charted reef on a lovely, blue-sky day with no wind to speak of?"  The owners of the ship, alas, have not been very forthcoming about how they managed this little bit of nautical incompetence.  But this could be a case where a large ship did indeed go aground due to excessive trust in GPS. 

Bob

p.s. Given that the BVI's were last charted in the 1800s, it is really rather remarkable that they are ONLY 40 or 50 yards off the mark.  There were some pretty serious navigators and cartographers back in that era.