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Hi Gary,
No problem today:
Re: Dip-meter again
From: Gary LaPook
Date: 2012 Apr 9, 03:55 -0700
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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.

gl
Regards, Noell
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