# NavList:

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

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Re: Problem
From: Bruce J. Pennino
Date: 2013 Apr 11, 09:20 -0400
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Hello Noell:

I just took a "Google" look at your breakwater. I'm surprised about the large difference between your estimated HoE and what you finally determined as 22.5 ft.  Have you reconciled the difference between 15 and 22.5 ft. ?

The breakwater is a typical Corps breakwater and the rip rap is probably on a slope of 1 vertical to 1.5 horizontal, maybe 1 V:2H. Stone size is 2-3 ft more or less; there is a sloping slab with a light at the tip , right? Slab maybe 8 ftx 8 ft.

The next time you go there, and someone is standing on the breakwater and you are on the shore, could you reestimate the number of " person heights" of the top of the breakwater to the water level, then add your height. Good comparison to 15 or 22?

I discovered last night "Google Maps Distance Calculator".  You can use this online calculator to get
the " straight " line distance between any two points.  Very easy, even for me with modest computer skills. I assume that distances (miles) are really a great circle distance (probably statute miles), which is easily calculated by hand with a calculator or online by using latitude and longitude of the points.  I believe great circle distances are traditionally given in NM, but as always, must be careful with defining terms.  Later today I'm going to measure my driveway  width and other longer distances and compare to "Google Maps Distance Calculator".

Now I no longer need to scale distances..... just "click"..."click".  Very cool. Check out the channel width of  512 ft.

Best regards,

Bruce

----- Original Message -----
Sent: Wednesday, April 10, 2013 1:04 PM
Subject: [NavList] Re: Problem

Hi Gary,
No problem today:
Re: Dip-meter again
From: Gary LaPook
Date: 2012 Apr 9, 03:55 -0700
[Previous message] [Next message] [Reply to this message]
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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