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    Re: marine sextant on land
    From: Frank Reed
    Date: 2008 Oct 30, 00:50 -0400

    Paul H, you wrote:
    "In some directions I have a reasonable horizon -- mountains in the
    background but flat land nearby. But mostly the view is obstructed by
    buildings, trees, and a fence. That's all right. I already know my position,
    so the altitude intercept directly indicates the error in my horizon
    estimate. From that I'm learning where the horizon lies on these
    You can also use converging lines on any building, road, or other structure
    that's been constructed horizontal. For example, suppose you have a
    multi-story that's been built recently and presumably with proper verticals.
    As you look down the length of the building, the lines of the floors and
    windows extended appear to converge at some point in the distance. That
    point rests on the true horizon. If you take a photo of the scene and
    carefully draw lines along those building lines, you can find the exact spot
    on the background mountains or whatever is in the distance. From then on,
    you have an accurate horizon for that spot. Even train tracks or rows of
    street lights will work in areas where the land is flat, but don't use lines
    farther away than a couple of thousand feet since at greater distances the
    curvature of the Earth comes into play.
    Incidentally, instead of finding the horizon, if you live among tall
    buildings, you can also find the zenith with fairly good accuracy by
    extrapolating the converging point of vertical lines (but be aware that some
    tall structures are built with slight convergence anyway for esthetic
    architectural reasons and then the trick doesn't work).
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