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» Online Classroom   » Inland and Coastal Navigation   » Public Discussion of Inland and Coastal Navigation   » Using Mark 3 Sextant for Coastal Navigation

Author Topic: Using Mark 3 Sextant for Coastal Navigation

 - posted December 08, 2006 05:50 PM      Profile for MTrainor           Edit/Delete Post 
I have a question about using the Mark 3 Sextant for coastal navigation. In the product description the statement is made..."it is actually the sextant of choice for inland and coastal piloting. Using the sextant horizontally, you can find your position on a chart much more accurately than you can by other conventional piloting techniques such as compass bearing fixes."
How this is done?

From: Mesa, AZ
David Burch

 - posted December 09, 2006 02:03 PM      Profile for David Burch           Edit/Delete Post 
thanks for the question. i am going to extract a couple articles from a recent edition of Navigator's Newsletter which has a good summary. This is great motivation to do this, which we have been meaning to do for some time. be back shortly. --david
From: Starpath, Seattle, WA
David Burch

 - posted December 09, 2006 03:02 PM      Profile for David Burch           Edit/Delete Post 
You can now download an article (a small elibra file) on the use of Mk 3 sextant for horizontal sextant angles. This article and a couple related notes have been extracted and reproduced with permission from the quarterly newsletter of the Foundation for the Promotion of the Art of Navigation. (Starpath is the all-volunteer editors of this quarterly publication.)

To read this you will need to install a copy of the free elibra reader. Printed copies are available to Foundation members.

The article covers horizontal angles solved by a combination of plotting and a short computation. In our home study course, we include a method of plotting alone without computation.

The value of this method is broad. We cover some uses of it in the Emergency Navigation book... just substitute the makeshift measurements we present there with the more accurate ones you get from the sextant. That book also presented some short cut methods of analysis to get distance off, but there are more accurate solutions.

You can use sextants to get distance off or a circle of position... or as the article here shows, a very precise fix. The angles measured can be horizontal or vertical (both are covered in the Emergency Nav book.

The more exact solutions for the vertical angles are given in Bowditch, and we have compiled a calculator set (originally from NIMA) that solves all three precisely. You can download this from the Freeware section of our downloads page.

You have three vertical angle measurements to consider. Base to tip (assumes you are seeing the true shoreline below the land in front of the object); Waterline to tip (implying the true base of the object or shoreline in front of it is below the horizon); and waterline to horizon above the waterline... that is, the suppression of the waterline below the horizon.

This last method is very interesting but it does require an accurate sextant. Might be able to do it with the Mark 3, but must be careful, as you are often dealing with just minutes of angle here. More on this technique later, which can in fact be used to measure how far off a boat is by measuring the angle between its waterline and the horizon behind it. You must in this case take into account your height of eye.

The explanation of the calculator solutions in Bowditch are in that book, which can be downloaded online or purchased in print form.

There is a lot to be discussed about sextant piloting. If further questions arise, please post them here. See for example this article here on how to take horizontal sextant sights.

You will also see in the Newsletter article by John Hocking the reference to a 3-arm protractor (he doesn't like them!). They provide a way to get the fix without any computation or plotting. They are getting harder to find these days. The plastic ones we carry actually do a very nice job.

We have made a presention at the ION Virtual Museum of a very good one we have at the school. The official name of a 3-arm protractor is "Station Pointer."

From: Starpath, Seattle, WA

 - posted December 12, 2006 05:18 PM      Profile for MTrainor           Edit/Delete Post 
David, Thanks for the most informative article. Is there any way you can send it to me in a printable form? I'll need to review it out on the water as I take sights and do the computations to learn the technique. I was hoping to have a hard cpoy of the article to refer to at that time.
Thanks, Mark

From: Mesa, AZ
David Burch

 - posted December 12, 2006 07:27 PM      Profile for David Burch           Edit/Delete Post 
The best way to get a reprint of that unique article, and others like it, is to join the Navigation Foundation then that back issue can be included in your welcoming package. The annual membership dues is $35 but you get tremendous discounts on all navigation publications and books, including starpath software and courses (ordered through the Foundation). It is a worthwile organization and serves a good cause. With out the hard work of such organizations, we might all end up pushing buttons to navigate and just hoping it all works right. We support them in every way we can.

Please see the write up we have on them on the archive page (we donated the work on that project)

From: Starpath, Seattle, WA

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