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Author
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Topic: Shoreline for a practice horizon
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David Burch
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posted February 11, 2005 09:14 AM
Forwarded from Helpdesk.
I am reacting to David's suggestion that I could use the far shore of the Chesapeake Bay as a substitute for the real horizon. I searched your Starpath Library for a related article but found none.
I am looking for the instructions on how to use a substitute horizon and how to compute the height of eye from a position on shore 10 to 20 feet above the water line. Then there is the tide issue. The water level at the shore has a normal range of about 4 feet but during storms and heavy wind days it can be as much as 10 feet.
Any ideas on a resource for these questions? Thanks
From: Starpath, Seattle, WA
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David Burch
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posted February 11, 2005 09:18 AM
This subject -- called dip short -- is covered in detail in our course on cel nav, Chapter 2 and links to topics in the back.
We do have a discussion of the tide issue somewhere online in one of the links from the plastic sextant article, wherein i was responding to an article in Cruising World that did not take into account the tide in a similar measurement. i think it is plastic sextants, practice sights, cruising world. If you cannot find it from that path, please let me know.
Dip short is also covered in Bowditch, but the tables they provide are too coarse. We need to use the formula presented in our text book.
We have used this technique extensively and it works well providing the input is correct.... ie Height of eye and distance to the horizon must be right.
Do a search of this forum on "dip short" to see other discussion, and also i would recommend signing up for the online course and then also search the Student Discussion Forum as well.
From: Starpath, Seattle, WA
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south
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posted March 03, 2007 04:06 AM
I have tried the dip short of the horizon method several times for practice from a beach. Using my real position as my assumed position I generally find an error of typically about 150 metres from my true position by using a shore two miles away. I've done this at night without being able to judge where the shoreline actually is with precision, but still get good accuracy. Very handy method.
From: Australia
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David Burch
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posted March 03, 2007 08:26 PM
Excellent. thanks for the follow up. This is a good trick to know.
I must note that 150 meters accuracy is 150/1852 = 0.08 nmi precision. So we must keep in mind that the nautical almanac data we use in cel nav is only accurate to 0.1' so when we get this type of agreement we must be (a) proud of our work, but also (b) mindful that there is some luck involved. In short, if we only agreed to within 0.2' or 370 meters we should be equally proud, and in fact we should in practice push this to 0.3 to be very happy with our work and even even 0.4 or 0.5 when we are underway.
And I refer here to sights with real horizons, not dip short. We have seen dip short come out very well, as observed here, but generally you have to give yourself a bit more leeway with these sights.
From: Starpath, Seattle, WA
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south
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posted March 04, 2007 10:24 PM
I should have said that my intercept is typically 150 metres.
I find that if I get the sextant as close to the water as possible, no more than 1 metre, then pin-pointing where the exact shoreline is, isn't as important. I got an intercept of several hundred miles one night, but when I got home and worked it out with a nav program I found that I had named the wrong star which brought the intercept back to a normal range!
The dip short method is excellent.
From: Australia
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David Burch
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posted March 04, 2007 11:00 PM
I misunderstood. Sorry. Yes, if you have a convenient place with a nice rock right on the beach and you have good boots, you can sit right down with your feet in the water and a small lake will be a good horizon without dip short!
From: Starpath, Seattle, WA
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