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Old 02-04-2016, 16:08   #1
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Google tells me line of sight

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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Old 02-04-2016, 16:29   #2
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pirate Re: Google tells me line of sight

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..
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Old 02-04-2016, 16:47   #3
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Re: Google tells me line of sight

most definately, pigs on a beach 5 miles away might appear to be flying ...
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Old 02-04-2016, 17:16   #4
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Re: Google tells me line of sight

You can see more than 3 miles but the horizon is only about 3 miles away because of the curvature of the earth. If something is off the water like a mast, airplane or ships superstructure, it can be seen from quite a distance away. Saw the Big Island when closing Hawaii at about 30 miles but that was because of clouds and haze over the Island. Probably could have seen it at 50 miles since it sticks 13,000 feet in the air if visibility would have been better.
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Old 02-04-2016, 17:26   #5
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Re: Google tells me line of sight

Line of sight on the ocean and elsewhere on a cloudless day is quite a few light years away.

However the horizon , if your height of eye is about 2 metres, is about 3 miles.. i.e. you would be able to see a beach ball floating on the horizon...given 20/20 vision and a pair of 10 x 50 Fujinons.

If there was someone else looking at the beach ball from 3 miles away on the other side , that is 6 miles from you, you would be able to see his hat.
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Old 02-04-2016, 21:51   #6
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Re: Google tells me line of sight

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
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Old 03-04-2016, 06:27   #7
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Re: Google tells me line of sight

I think the confusing part is you are thinking of objects that extend above the waters surface. Those can be seen from much further away (depending on the object height)

The google example is the actual water surface visibility (presumably assuming no waves).
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Old 03-04-2016, 07:10   #8
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Re: Google tells me line of sight

No...not thinking I am at water level but stating up on the deck of a 30' boat. Probably 3 feet above the water. Eyes 8' above the water. Google tells me I can see approx. 3 miles.

I like the formula that was supplied in a previous post.

Now I am trying to back into the curvature of the earth with math. Just to double check. Using circumfirance of the earth as 24,901 statute (I think) miles.

Any math majors out there?

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Old 03-04-2016, 07:19   #9
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Re: Google tells me line of sight

Quote:
Originally Posted by tuffr2 View Post

Any math majors out there?

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You're reinventing the wheel (pun intended ).

In addition to the handy formula previously provided, any qualified reference text on nautical navigation will include this information and show you the derivation.
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Old 03-04-2016, 07:23   #10
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Re: Google tells me line of sight

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I think the confusing part is you are thinking of objects that extend above the waters surface. Those can be seen from much further away (depending on the object height)

The google example is the actual water surface visibility (presumably assuming no waves).
Use the forumla twice, once for your height, once for the height of the object at which you are looking, add the two together.

Or use this nomogram

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Old 03-04-2016, 07:35   #11
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Re: Google tells me line of sight

All this started when my friend asked...how far away is that mountain. I said 30 statute miles. Then at the ocean someone asked how far away is that sailboat. I said I do not know...thus my Google search.

Now I am surprised to know you can only see a few miles on the water because of the curvature of the earth. Kinda neat stuff to know.

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Old 03-04-2016, 07:41   #12
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Re: Google tells me line of sight

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Originally Posted by tuffr2 View Post
Now I am surprised to know you can only see a few miles on the water because of the curvature of the earth. Kinda neat stuff to know.
WAIT!!! It's NOT flat???

Where'd all those submarines go???
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Old 03-04-2016, 08:04   #13
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Re: Google tells me line of sight

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Originally Posted by tuffr2 View Post
All this started when my friend asked...how far away is that mountain. I said 30 statute miles. Then at the ocean someone asked how far away is that sailboat. I said I do not know...thus my Google search.

Now I am surprised to know you can only see a few miles on the water because of the curvature of the earth. Kinda neat stuff to know.

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This is exactly the situation I and a few others have described. The mountain and the sailboat both extend well above the surface of the earth and thus can be seen for much further than the actual surface of the water.

Thought experiment (ignoring curvature of the earth): Find a 5' fence and place a tennis ball and a lawn chair 5' on the far side of the fence. Now slowly back up until you lose sight of the ball. Then back up until you lose sight of the chair.

Assuming you are fairly tall and have a 6' eye height you will lose sight of the ball at just over 6' 3" from the ball (1'3" from the fence).

Assuming the chair is 4' tall, you won't lose sight of it until you are 10' from the chair.

The curvature of the earth makes the math a little more complex but the concept is the same.
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Old 03-04-2016, 08:18   #14
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Re: Google tells me line of sight

Use a drone
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Old 03-04-2016, 08:32   #15
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Re: Google tells me line of sight

A publication named after Nathaniel Bowditch has many wonderful tables that can help with many questions that a mariner might have while at sea, think of it as google of the maritime book world.
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