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Sorry I forgot to provide before some information about the two photos
with the large difference in terrestrial refraction. They were taken
at two locations separated by about 500m but from both locations the
island has the same distance. The HoE was for the first photo an
estimated 3.5m and for the second an estimated 4.5m (thus contrary
from what one would conclude from the photos).

Marcel


On Sat, Apr 13, 2013 at 11:46 AM, Marcel Tschudin
 wrote:
> 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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>> : http://fer3.com/arc/m2.aspx?i=123548

   

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