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    Re: Dip-meter again
    From: Alexandre Eremenko
    Date: 2012 Apr 9, 15:01 -0400

    Pity you had no way to measure your height of eye.
    On Mon, 9 Apr 2012, Bill Morris wrote:
    > Kermit and Alex
    > Attached is a small spreadsheet of three sets of observations of dip made 
    from Henderson Bay in New Zealand at 34deg 44.27minS , 173deg 06.6 min E. The 
    azimuth 0 is a few degrees east of north. The horizon of azimuth 120 is 
    before land about 25 km away. The approximate height of eye was 22 metres. I 
    used a Wild T2 theodolite that reads to single seconds, but I do not think 
    single observations are likely to be accurate to that precision. Seen through 
    the 26 power telescope, the horizon is not a straight line but there are fine 
    waves visible and on occasion the horizon appeared to move up and down a few 
    arc seconds over a period of a few seconds. When this happened I checked the 
    bubble for movement in case I had inadvertently disturbed the theodolite. I 
    had not. In any case I checked the bubble for coincidence before and after 
    making a reading. I think +/- 5 seconds would be a fair guestimate for the 
    accuracy of the observations.
    > On 8 April, the horizon appeared sharp to the naked eye and there were many 
    fair-weather cumulus clouds in the sky. On the 9th the horizon was again 
    sharp and the sky was cloudless. Air temperature on both occasions was about 
    22 degrees. I did not measure the sea temperature. There was a light breeze, 
    certainly not enough to move the theodolite, Alex.
    > Plainly, the dip can vary from day to day and minute to minute, but with the 
    limited data I can provide, I do not think one can comment about azimuth 
    dependence. The observations do appear to underline that variable refraction 
    sets a limit to the accuracy of altitude observations at sea.
    > Bill Morris
    > Pukenui
    > New Zealand
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    > Attached File: http://fer3.com/arc/img/118849.dip.xlsx
    > View and reply to this message: http://fer3.com/arc/m2.aspx?i=118849

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