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    Re: Dip-meter again
    From: Marcel Tschudin
    Date: 2013 Apr 13, 11:46 +0300

    A precise horizontal distance alone is not sufficient for observing
    the dip variations. You also need a "fixed" horizontal reference line,
    e.g. a fence, a border or a roof within a short distance e.g. 100m to
    300m where refraction is likely still negligible.
    For my observations I did not have a horizontal reference line. In a
    few cases I could however observe a remarkable change in refraction
    between horizon and a feature protruding from behind it. The attached
    picture shows two such situations where the apparent horizon differs
    by about 5 moa compared to the island behind it.
    Victor Reijs shows on his Web-page here
    http://www.iol.ie/~geniet/eng/refract.htm#limiting a plot with
    published refraction measurements near the horizon. One series of
    measurements (Seidelmann) show a systematic difference between sunrise
    and sunset of about 6 moa. This difference may be specific to the
    location where the measurements were made.
    On Sat, Apr 13, 2013 at 6:25 AM, Bruce J. Pennino
    > ________________________________
    > Hi Frank and All:
    > Frank, you've  mentioned this thought (vanishing buildings) before. I hate
    > to admit it, measuring refraction in this manner is intriguing.  Maybe my
    > next thought fits. Now  that we know that we can get precise horizontal
    > distances between objects (I'm now thinking tall slender building or towers
    > in a row.....oil rigs or wind turbines). We also know that with my common
    > theodolite I can measure vertical angles to 3 seconds or so of vertical arc.
    > I really don't believe +/- 3 seconds because I still don't have operator and
    > collimation errors totally sorted out.  Say I really can confidently measure
    > to +/- 10 seconds.
    > I could  set up  someplace where I can see these two or three
    > buildings/towers  several miles apart.  How much does the meteorological
    > conditions have to change for me to measure the CHANGE in refraction based
    > on the apparent  change  in vertical heights of the building? Does the
    > temperature have to change 30F; weather front come through going from
    > relatively low atmospheric pressure to high pressure; where does relative
    > humidity come in?  How about the azimuth of viewing; time of the year, angle
    > of the sun , and I'm sure many things I have not considered? Doable?
    > Or, how about setting up a camera  with a special lens or a stadia type
    > attachment and monitoring the buildings . Monitor weather at the same time.
    > By stadia eyepiece I mean several horizontal index lines or equivalent.
    > Knowing the height of the camera and buildings, and changes in apparent
    > height of the buildings, I believe you wrote that  we  could calculate
    > refraction change.  Just thinking and trying to relate your thoughts and
    > those presented by Marcel. Seems difficult to quantify?
    > Probably should rename this topic?
    > Bruce
    > ----- Original Message -----
    > From: Frank Reed
    > To: bpennino.ce---net
    > Sent: Friday, April 12, 2013 1:53 PM
    > Subject: [NavList] Re: Dip-meter again
    > ________________________________
    > Gary, you wrote:
    > " I realized that I could get an accurate measurement of the width of the
    > channel by using Google Earth and that I could measure the angle below the
    > horizon to the waterline on the opposite breakwater and with this
    > information calculate my accurate height of eye."
    > If I've understood your description correctly, essentially you're using "dip
    > short" to get height of eye. And yes, this works exceptionally well in cases
    > like this where you can figure out the exact linear distance to some feature
    > with a clearly defined waterline.
    > Next, suppose you have several objects with clearly defined waterlines at
    > exactly known distances between you and the horizon (ideally, these would be
    > at regular intervals, e.g. a mile apart). If you measure the angles from
    > their waterlines to the horizon, it should be possible to solve for height
    > of eye AND the terrestrial refraction constant k (the rotation of a light
    > ray in minutes of arc per nautical mile). And if you return to the same site
    > under different weather conditions, you should find that the angles change
    > as k changes. Visually, the more distant objects would appear to group
    > together or spread apart vertically as the refraction changes. I'm still
    > hoping someone will make a great time-lapse video of this showing the
    > refracted view of objects towards the horizon breathing in and out during
    > the course of a day.
    > -FER
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