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Author
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Topic: Dip error bay of fundy
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Capt Ahab
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posted January 24, 2013 09:27 PM
apologies in advance if necessary for my question if not appropriate (new Here). I am a mechanical engineer and licensed master who owns a small coastal vessel for diving and lobster, fish,sealwatching, etc.in Noank ,CT. I also was a sailor in my younger days as well, and wish I could do both. I am however determined to maintain the old time skills being lost to time, including celestial navigation.
Anyway, this one has bugged me for awhile: If I stand on a cliff overlooking the bay of fundy, and a man on a vessel is at my height next to the cliff, we have the same dip correction.
If I stand on the same cliff at low tide, and the man is now 50 ' below me still afloat. And I correct Hs for dip (e.g. add the additonal 50' to my original correction), and his dip is unchanged e.g.same deck, same man, same boat, same ocean.
Our dip corrections are very different, but do we get the same positional answer assuming identical sights otherwise?
Perhaps a bit of a dumb question,as open sea navigation is comparatively free of such extreme differences, which are more earth/shore contour related. On the other hand, the closer I come to a shore, the more accurate I would prefer to be, and the more effect land contour has on the observed height of the water.
The "sloshing bowl" theory of incoming and outgoing tides (such as described in Eldridges and others) would suggest that in coastal regions the differential would effect the dip correction perhaps with some signifigance as I am looking "downhill" or "uphill" at the open sea, which is creating the basis for the directional flow and rapid tidal movements in/out in extreme tidal areas.
From: Noank, Ct
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Capt Steve Miller
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posted January 26, 2013 09:15 PM
Sit tight I am looking into this.
From: Starpath
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David Burch
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posted January 27, 2013 01:42 AM
While this is being checked, i might note that dip angle depends on the height of eye above the water. if that changes, the dip changes by just that amount. it also assumes that the sea level is the same where you measure it and where the horizon is, but that is a fair assumption since the horizon is just some 7 miles away in your example.
thus for the dip just look it up in the normal manner for your height.
if the two of you were next to each other, one 40 ft lower, he would read a lower Hs angle than you at the same moment, and be using a smaller dip but your Ha would be the same after dip correction, and you would each get the same LOP.
maybe i miss the point of your question, but that is how i see it. --david
From: Starpath, Seattle, WA
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Capt Steve Miller
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posted January 27, 2013 10:25 AM
Your altitudes (Hs) will be different and Ha will be different due to the Height of Eye (dip) correction, BUT your Observed Altitude (Ho) will be the same.
From: Starpath
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Capt Steve Miller
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posted January 28, 2013 09:55 AM
I'd like to revise my previous post as I passed along some wrong info. Your Hs will be different, your Ha will be the same due to the correction for dip and your Ho will be the same.
Sorry if I caused any confusion.
From: Starpath
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Capt Ahab
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posted January 31, 2013 08:54 PM
many thanks, seems to make logical sense. i was wondering if the extreme tides made the ocean out of "sea level" enough to affect the readings
From: Noank, Ct
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David Burch
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posted January 31, 2013 09:43 PM
Sea level is gradually rising. This is a fact. But the tide tables are always referenced to the existing known mean sea level. So changes in maximum tides does not change anything we do in cel nav. Once you are out on the water all of these issues go away completely. At sea you are taking all sights relative to the sea horizon, and for this we do not care at all about the absolute value of the sea level.
... not sure i answered your question, however.
From: Starpath, Seattle, WA
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Capt Ahab
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posted February 03, 2013 11:44 AM
I guess I overthink things. Eldriges tide and pilot book has an illustration of tides(page 155 of 2012)called "relationship of high water and ebb current". Basically tides in certain coastal regions form waves as they are funneled and "slosh" into estuaries and bays. Bowditch 2002 would note this height difference as a force which can cause standing waves at inlets and estuaries.
Basically it theorizes that water comes in as a long wave .... driven by height difference caused by tidal forces and distorted upward by land shape and funneling effect. The wave is created by the contour of the land so the water stands up into this Long incoming wave. Hence my goofy worrying about the effect on dip, as the picture would imply that in extreme tidal zones the effect would be a distortion of mean sea level for some unknown miles based on a rush of water into a restriction creating this theoretical wave. In other words..a distortion implies that any long wave would also have a long trough, thus distorting the view of sea level loking out across the horizon. If I stand on the crest looking out across the trough after the offshore tidal flow has reversed and started ebbing my mean sea level view is downhill because the momentum of water into the estuary or bay may not yet have dissipited and i may still be at the end of flood. This becomes why tide changes at different times in local regions...some of the water is still coming in after farther out the flow has reversed...and we have areas here where the reversal of the tide is an hour or more different within a 8 miles... In real extreme areas i theorized that this could distort dip...
Sorry for asking such an esoteric question...in that it would be so limited in application to coastal regions with extreme tides and contours... when I think through all the replies I realize that it is almost a pointless concern.... if I was standing on a tidal bore...or peak of this long tidal wave in fundy on an extreme section of coast I would most certainly be in "piloting" waters near land, and I certainly at that point better know where I am more accurately than my sextent can indicate.
obviosly as noted, the angles of dip and altitude would always normally be self correcting on a mean sea level, based on the difference in height of the observer.
thanks again for the responses as it helped me think it through. I tend to overanalyze as I am also a mechanical engineer...and we tend to want to see how all the pieces fit together.
From: Noank, Ct
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David Burch
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posted February 03, 2013 12:27 PM
The relationship between tide and current is a complex one, with the patterns varying somewhere between standing wave tide and progressive wave tides. These two patterns have exactly opposite behavior. In the Pac NW we have areas 40 miles apart with completely different behavior... ie peak flow occurring at High and Low water vs peak flow occurring halfway between High and Low water.
We must be very careful in generalizing this behavior. We discuss these matters in detail our textbook Inland and Coastal Navigation, which is now available in Kindle format readable on any computer or tablet.
From: Starpath, Seattle, WA
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