T&T: Aids to Navigation

[LRZeitlin@aol.com]

In a message dated 6/21/07 12:00:43 AM, Bob writes:


> Without a chart (or forward looking and side-scanning sonar) how are you
> going to know what is in the next yard ahead of you without some sort of
> chart (as inaccurate as it may be) or a navaid (better than nothing)? A
> depthfinder shows the depth where you are currently; your eyes can't see
> anything (for the most part in many waters) below the surface. Waves
> crashing over a bar or current swirling around an object below the
> surface are obvious indicators of bad things in the water, but those
> types of indicators don't occur very often.
> 
> 

Bob,

I don't mean to dis all charts and ATONs but it is wise to remember what 
charts are. They are graphic representations of an underwater portion of the 
earth's surface based, for the most part, on statistical sampling. ATONs are 
generally placed to mark hazards or indicate channels for commercial shipping. 
Shorelines are mapped by traditional surveying methods aided, in the last half 
century, by aerial photography. For the most part, charted hazardous objects are 
not located by much more precise GPS readings.

My point was that charts are updated less frequently than nature changes the 
bottom contour of inshore and coastal waterways. What might have been a 
reasonable approximation of the bottom decades ago may be much less reasonable 
today. I used to worry about this especially when I noticed that some of the charts 
I was using for the ICW were last updated in 1947. A tug captain told me not 
to worry. "The ICW changes with every storm. Because the route is constantly 
silting in and being dredged, any years chart is as inaccurate as any other."

In my case, my boat draws only 4 feet. Depths beyond the four foot contour 
are irrelevant unless I am looking for an anchoring location. ATONs, placed for 
big ship navigation, show me areas to avoid. The big ships can't go where I 
can go. My cruising speed is less than 7 knots so I have plenty of time to make 
decisions. When I approach an unfamiliar shoreline, I go dead slow with one 
eye on the depthfinder and the other on the water just ahead of the bow. If I 
strike anything it is unlikely that I will cause any damage.

But I always keep in mind that a chart is a guide, not a contract. If you hit 
an uncharted underwater object, you can't sue NOAA. Read the fine print.

As an aside, Columbia University is conducting a detailed underwater survey 
of the Hudson River basin using ultra precise side looking sonar. They have 
uncovered hundreds of underwater objects not included on official charts. Some 
are granite spurs not leveled by the last glacier, some are old shipwrecks. Some 
the foundations of concrete piers and structures, Some are railway cars that 
fell off barges during storms. The big ship channel is largely free of these 
objects but they litter the shoreline waters.


Larry Z


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