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Bruce,

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.

Marcel





On Sat, Apr 13, 2013 at 6:25 AM, Bruce J. Pennino
 wrote:
> ________________________________
>
> 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
>
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> ----- 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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