# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Dip-meter again
From: Bruce J. Pennino
Date: 2013 Apr 12, 23:21 -0400
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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
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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