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    Re: Dip-meter again
    From: Gary LaPook
    Date: 2012 Apr 9, 03:55 -0700
    As long as we are talking about improving our dip corrections I would like to report on something I tried last week. The usual location I go to to take observations with  my marine sextants is on the south side breakwater at the entrance to Channel Islands harbor at 34° 09.4' N, 119° 13.5' W. I have attached a Google Earth picture of this location. I have always just estimated my height of eye as 15 feet but it is difficult because I could not just drop a measuring line down to the water level because the sides of the breakwater are not vertical.

    This time I also brought my A-7 bubble octant because it also has the capability to take observations from the natural horizon and I wanted to try out this capability. Standing there I got an idea. 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. Using the A-7 I measured minus 2° 31' and I lined up the structures in the background so that I could be sure that I was measuring the width of the channel in the right direction.  I then took my series of shots and when I got home I pulled up the location on Google Earth and measured the width of the channel as 512 feet which make my height of eye 22.5 feet. I have attached a second Google Earth image showing where I measured the width.

    Your next question, why not just measure the angle to the horizon and determine dip directly instead using the height of eye to enter the normal dip table? Well, I did that but I only had time to take five readings and  the readings have quite a bit of variability so I think the first method would produce a more accurate dip value. The next time I will experiment and take many altitudes of the horizon to see how accurately I can determine dip directly. Obviously an aircraft bubble octant lacks the precision of your theodolite.


    --- On Mon, 4/9/12, Bill Morris <engineer{at}clear.net.nz> wrote:

    From: Bill Morris <engineer{at}clear.net.nz>
    Subject: [NavList] Re: Dip-meter again
    To: NavList@fer3.com
    Date: Monday, April 9, 2012, 2:16 AM

    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
    New Zealand
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