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Old 03-04-2016, 08:46   #16
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Re: Google tells me line of sight

Quote:
Originally Posted by tuffr2 View Post
With Google there really is not much that one can not find out. But none the less of is hard to believe you can only see 2 - 3 miles.

For some reason without trees/ mountains in your way I would have thought the line of site on the ocean would be 25 -30 miles. Not 2 - 3 miles.

Does that surprise anyone else?

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This is an example of the things we don't always think about... and you now have your answer.

For example, I had some friends sailing with me for a few days years ago in Prince William Sound [ocean waters...] and they were both engineers.

One afternoon, while enjoying the 5000+ foot mountains surrounding the fjord we were in, one casually asked me what our altitude was. I looked over at them with my best deadpan face waiting for the punch line, but it was not forthcoming. Seizing the opportunity, I grabbed the tide tables, did a quick rule-of-twelfths calculation in my head, and replied seventeen feet; give-or-take.

She immediately blushed. We all had a good laugh, and then I answered the intended question: The mountains with the rounded tops are under 5000 ft because that was the height of the ice in this region during the last ice age... Those with jagged tops are over 5k ft...

Regarding the horizon, I tell guests to just assume their eyeballs are 9 ft above the water, and take the square root of that for a close enough estimate. There is your 3 miles to the horizon... [Another reason to use AIS at sea; to avoid the other moving parts...]

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Old 03-04-2016, 09:10   #17
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Re: Google tells me line of sight

Lol...yep, engineers ask these type of questions. I have a friend that is a civil engineer. Lots of distance and elevation gain/decrease questions.

Standing at a beach looking out over the ocean is 2 miles and at an elevation of 9' is 3 miles. I think I know something he may not know...



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Old 03-04-2016, 10:25   #18
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Re: Google tells me line of sight

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Originally Posted by boatman61 View Post
If you look at the horizon very carefully on a good visibility day you'll maybe notice an ever so slight curve either edge of your vision.. one of the reasons from a 9ft eye elevation one can almost exactly calculate a ships distance.. as soon as you make out the waterline its 3 miles away and closing... 20/20 vision only..
When you can make out the waterline, a ship is to damn close.
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Old 03-04-2016, 11:06   #19
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Re: Google tells me line of sight

Quote:
Originally Posted by tuffr2 View Post
With Google there really is not much that one can not find out. But none the less of is hard to believe you can only see 2 - 3 miles.

For some reason without trees/ mountains in your way I would have thought the line of site on the ocean would be 25 -30 miles. Not 2 - 3 miles.

Does that surprise anyone else?

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From cockpit height (6ft) the horizon is 3 miles due to earths curvature and sand on the beach becomes visible at 1 mile.
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Old 03-04-2016, 11:18   #20
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Re: Google tells me line of sight

? On the crest or in the trough.
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Old 03-04-2016, 15:36   #21
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Re: Google tells me line of sight

Bowditch, table 12
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Old 03-04-2016, 15:45   #22
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Re: Google tells me line of sight

try that again, for some reason it did table 13, which can be used but takes a bit.

Here is table 12
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Old 03-04-2016, 20:48   #23
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Re: Google tells me line of sight

Quote:
Originally Posted by wrwakefield View Post
This is an example of the things we don't always think about... and you now have your answer.

For example, I had some friends sailing with me for a few days years ago in Prince William Sound [ocean waters...] and they were both engineers.

One afternoon, while enjoying the 5000+ foot mountains surrounding the fjord we were in, one casually asked me what our altitude was. I looked over at them with my best deadpan face waiting for the punch line, but it was not forthcoming. Seizing the opportunity, I grabbed the tide tables, did a quick rule-of-twelfths calculation in my head, and replied seventeen feet; give-or-take.

She immediately blushed. We all had a good laugh, and then I answered the intended question: The mountains with the rounded tops are under 5000 ft because that was the height of the ice in this region during the last ice age... Those with jagged tops are over 5k ft...

Regarding the horizon, I tell guests to just assume their eyeballs are 9 ft above the water, and take the square root of that for a close enough estimate. There is your 3 miles to the horizon... [Another reason to use AIS at sea; to avoid the other moving parts...]

Cheers!

Bill
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Old 04-04-2016, 00:47   #24
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Re: Google tells me line of sight

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Originally Posted by jackdale View Post
The formula:

1.17 times the square root of your height of eye above the water in feet = Distance to the horizon in nautical miles
For those thinking in metres, the formula is:
2.1 x sqrt (of height of eyes) + 2.1 x sqrt (of height of object)
height of eyes and height of object is in metres above sea level
the calculation will give visibility in NM.
"sqrt" means square root

I have attached an older excel spread sheet I made many years ago; this calculates all the distances, but I am sure googling will give you a calculator on the net somewhere...........
hmmmm just looking now:
How to Calculate the Distance to the Horizon - BoatSafe.com
And if one wants to get technical:
Distance to the Horizon
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Old 04-04-2016, 07:46   #25
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Re: Google tells me line of sight

I have used these formulas also for estimating the distance to seamarks. If you know the approximate height of a seamark, and you can see e.g. half of that seamark above the horizon, you can estimate the distance to it. You need to know your own distance to the horizon and the distance of the "below the horizon part of the seamark" to the horizon, and then reduce the latter from the former (to get the distance between you and the seamark).

Usually plotters are much better in estimating the distance and will quickly tell you all the distances. I never bother to make these calculations while at sea. But I have calculated some rule of thumb values (knowing the usual height of my eyes in the boat, and being able to estimate the height of horizon at some common seamark models). These values can be helpful especially when using binoculars (that make it otherwise more difficult to guess the distance).
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Old 04-04-2016, 08:05   #26
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Re: Google tells me line of sight

BTW - if you are determining the distance to land based object, such as a light station, you must account the state of the tide at the time you are determining the distance. Elevations are based on HHWLT. That really complicates matters.
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Old 04-04-2016, 08:06   #27
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Re: Google tells me line of sight

I would like to add Marine Radar to discussion. FYI
Radar & eyesight can see "beyond" the horizon,if atmospheric conditions are just right-clear visibility & a water vapor layer. It is called "Looming" & is similar to mirage.
https://en.wikipedia.org/wiki/Loomin...tion_phenomena
However,when conditions are "normal"-ie: when you really need your radar-two avg 40ft boats do well to see each other at 4-5NM.

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