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    Re: Dip by sextant and systematic error
    From: Frank Reed
    Date: 2013 Apr 2, 12:10 -0700

    Marcel, you wrote:
    "However, I think that the availability of environmental (atmospheric) data and of analytical tools have improved over the past 200 years. One may therefore hope to derive from a sufficiently large set of observations some improved estimation methods. "

    I've considered this, too. Certainly today we can do highly detailed numerical simulations based on much finer resolution models of atmospheric temperature variations. The environmental data, though, is not really suited to our task. To predict refraction between an observer 25 feet high and the ocean horizon "zone" centered some five miles away, we would need access to detailed temperature and pressure data for that path. There are definitely some basic cases that we can handle and that we can measure like the case of constant lapse rate that I mentioned previously. It doesn't even have to be purely linear as long as it the profile all along the path is reasonably similar. I do agree that we could generate "on-the-fly" dip tables for situations where we have an extraordinary amount of environmental data. I can't imagine any circumstance where this would have navigational value. It's interesting physics, but ever more removed from the topic of navigation.

    You added:
    "Even if one would not arrive at an improved estimation one could at least *show* how it scatters."

    I fully agree that measuring the scatter is useful. The observations ARE worth doing.

    By the nature of the experiments, every set of data would unfortunately be limited to a particular locale. The horizon phenomena that I see from my favorite observing locations here on Conanicut Island are distinctly different from the phenomena that I see from Stonington Point. The two places are separated by only 24 nautical miles. The former faces open ocean with moderately high land to the immediate west while the latter faces a nearly enclosed sound with land to the north. I would guess that the different geography of these places influences the layering of air over the water, but this isn't really any help in generalizing the results from one observing location to another.

    -FER


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