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Re: Optimal HoE?
From: Brad Morris
Date: 2019 Aug 20, 15:40 -0400

Hello Tony

Take a glance at the inside cover of your Nautical Almanac.

Find the Dip Table. It provides the correction in minutes for HoE, in meters.

Let us first start with the very low HoE

METERS Correction

1.0 -1.8

1.5 -2.2

2.0 -2.5

2.5 -2.8

From this we can observe as much as 0.4 minutes of correction for a HoE change of as little as 0.5 meters.

Let us next examine the high HoE

METERS Correction

19.3 -7.8

19.8 -7.9

20.4 -8.0

From this we can observe 0.1 minute breakpoints, as the HoE is changed. In this part of the table, we see a change 0.5 meters HoE resulting in only a 0.1' correction.

Next examine the very high HoE

METERS Correction

40 -11.1

42 -11.4

44 -11.7

From this we can observe a 0.3 correction for a 2 meter change in elevation, or 0.666 meters for every 0.1 breakpoint.

I stand by my assertion that the HoE assessment is more critical as the HoE is smaller.

You mentioned your HoE as 1 meter, which is at the very low end of the table, with the greatest sensitivity. But if you are off by some 50 centimeters to your precise HoE, you will get an error in dip correction. At one point, I considered obtaining a very expensive elevation measurement device which depended upon a sealed hydrostatic pressure change. It was extremely accurate, as memory serves me. The price stopped me, several hundred US dollars.

I'm not sure where the 5% figure comes from. I am sure Frank will help us out with his understanding. It doesn't seem to align with the NA.

Frank is obviously correct about anomalous refraction. If you are on shore observing over very cold water, there will be AR. As the land heats up, cool air is drawn onto shore by the updraft over land. The temperature of the water and boundary air is going to be a bug bear.

As you lower the HoE, you bring the horizon closer, further exacerbating the situation. The list has gone back and forth over waves. If the horizon is very, very close, you most certainly will see them. If the horizon is very far away, you will not. But under what context do you correct for them and how. The predominant advice is to ignore them and simply use the "average horizon". If you are afloat, you are advised to take your observation at the crest of the wave. How one is to then see the average horizon is beyond my understanding!! Suffice to say under large waves, you may be forced to accept greater uncertainty.

Brad

On Tue, Aug 20, 2019, 3:04 PM Tony Oz <NoReply_TonyOz@fer3.com> wrote:

Hello and thanks for the comments!
Dear Brad, I measure my HoE (when on the groyne) by looking at the people near me - the place is very shallow, when someone is standing in the water "abeam" of me - I ask for his height and estimate my HoE from that. The very last session was taken with the HoE equal to 1.3 meters - it was the shortest intercept. Before that I simply guessed my HoE looking down the boulder I was sitting on - the sessions with the assumed HoE of 1 meter and 1.5 meters were with bigger intercepts - so I thought there must be significant sensitivity to the actual HoE. But may be that was only a coincidence too.
Next time I'll take a gauge tape with me. :)
Regarding the tide - in my part of Finnish Gulf (60°7.1'N 29°56.5°E) it is barely noticeable - unless it is strong wind on or off the shore. I always try to measure from the water itself. When I do sights from the beach I estimate my HoE more crudely - I measure how many heights of a man's figure (standing at the water's edge) fit between the visible horizon and man's head - and I assume HoE from the average height equaling to 1.8 meters. Apparently, as Frank supposes - the error is less critical in that case.
Warm regards,
Tony
60°N30°E

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