# Why Sea Level sometimes gives an illusion being at height

## Main Question or Discussion Point

Why Sea Level sometimes gives you an illusion being at height like a small mountain? Specially when you are driving down of the mountain? Please find the attached image to see what I am referring to.

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It only appears to be at a height because your view is aimed downward as you go down the mountain. So in your field of view, the horizon is higher up than it would be if you were level. Imagine you are sitting in a classroom and looking directly at the blackboard. If you now look down toward the floor, the blackboard appears higher in your field of view.

rbelli1
Gold Member
Also remember you are on a sphere or at least a close approximation of one.

BoB

A.T.
Why Sea Level sometimes gives you an illusion being at height like a small mountain? Specially when you are driving down of the mountain?
If you are driving down a mountain, then it's not an illusion that you are on a mountain.

I have even stopped the car, stand still and took a look; still sea looks at height of mountain. Now if you consider the close approximation of being in spear or mountain and looking down (it seems way exaggerated compared to being at pole and being at equator; I am only at small hill !!!). That is why it is harder for me to understand that I am looking down that is the reason. I understand the classroom example, but I tried to level my head and eye towards the horizon. :(

sophiecentaur
Gold Member
Why Sea Level sometimes gives you an illusion being at height like a small mountain? Specially when you are driving down of the mountain? Please find the attached image to see what I am referring to.
Is it not just because our sense of what is actually 'horizontal' is not very well developed? There is really no reason why Evolution would have given us the ability to resolve such small angles. Certainly, nothing that we experience in a smoothly rolling motorcar will be familiar with our basic senses. Let's face it, it was only when Newton came on the scene that people had much idea of such things at all. We get all our clues from what we can see and that's dominated by nearby things.

CWatters
Homework Helper
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There aren't many visual clues to distinguish the difference between the two cases...

As per my view this become happened because of human senses are adapted for use on the ground, when you are driving down of the mountain you faced sensory illusions.

sophiecentaur
Gold Member
There are many examples of streams and canals that appear to be running uphill. There is a water channel (disused Victorian Industry) on Dartmoor (South West of England) which seems to be sloping uphill, whichever direction you view it from.

Clear dry day - possible temperature inversion. That could create a superior mirage and make the horizon look higher than it otherwise would.

Perhaps you can go to the location in Google Earth, make sure vertical exaggeration is 1:1.
Go down to street level in the same location and see if the horizon looks the same.

(I tried to see if I could discover the location from the clues in the image but all I concluded is that you are probably right handed, driving at just over 40 mph and listening to FM 89.5 and located on the West coast of the US driving in a roughly WNW direction in the late afternoon)

sophiecentaur
Gold Member
Go down to street level in the same location
But everywhere looks distorted and more or less horizontal. The camera was not designed to give topographical information - just to be wide angle and tolerant of the road slope.

Clear dry day - possible temperature inversion. That could create a superior mirage and make the horizon look higher than it otherwise would.

Perhaps you can go to the location in Google Earth, make sure vertical exaggeration is 1:1.
Go down to street level in the same location and see if the horizon looks the same.

(I tried to see if I could discover the location from the clues in the image but all I concluded is that you are probably right handed, driving at just over 40 mph and listening to FM 89.5 and located on the West coast of the US driving in a roughly WNW direction in the late afternoon)

You are about right for the location. It is San Diego, Delmar County, I took the exit High bluff drive towards the ocean. Anyways this is what I found on your super mirage theory.
http://www.dailymail.co.uk/sciencetech/article-2130598/False-wall-water-created-Fata-Morgana-mirage-hidden-iceberg-Titanic-late.html

I would like to know if there is anyway if I can confirm any of the theories. I will try the suggestions here. I will drive down on weekend again and see. Thanks.

The picture was taken on Del Mar Heights looking WSW towards the Nob Avenue intersection from about here:
32°56'54.48" N 117°15'34.97" W
The Street View image is on a more humid day and the horizon looks just as high if not higher. It is hard to compensate for the (presumed) greater height that the Street View camera took the shot from, and to be sure that the atmospheric conditions are different... but overall I don't think this view supports my suggestion that it was as a result of a mirage.

sophiecentaur
Gold Member
Well well.
But what do you call 'up hill' under those conditions? Could be a moot point.
I have to take issue with "what would happen of the Earth stopped spinning?" part of that article. It takes things in totally the wrong direction, imho. It's just another of those exercises in nonsense scenarios which would involve unbelievable amounts of Energy and Organisation to achieve. They should be throttled at birth, as far as I am concerned.
[Edit: Ye gods - I just re-read this. It sounds far more grumpy than I intended!!]

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If I'm with my feet up to my ankles in the water of the sea, on the water's edge, where is the sea level, exactly where?
How far is it from me?

At 5 km you can still consider the sea level relative to my feet?

That is, 5 km from me, can a point on the surface of the water still be considered sea level relative to my ankles?

sophiecentaur
Gold Member
If I'm with my feet up to my ankles in the water of the sea, on the water's edge, where is the sea level, exactly where?
How far is it from me?

At 5 km you can still consider the sea level relative to my feet?

That is, 5 km from me, can a point on the surface of the water still be considered sea level relative to my ankles?
The Earth is a Spheroid so any distance away from you, along a tangent will be 'above' the sea. Assuming the UK is about 1000km long and the Earth's radius is 4000km, that means the extremities will be about 30m 'below' a tangent to the mid point. What would you call 'level'? The sea would not flow away from that mid point (ignoring tides and spin etc. etc..

The Earth is a Spheroid so any distance away from you, along a tangent will be 'above' the sea. Assuming the UK is about 1000km long and the Earth's radius is 4000km, that means the extremities will be about 30m 'below' a tangent to the mid point. What would you call 'level'? The sea would not flow away from that mid point (ignoring tides and spin etc. etc..
Maybe you want to say that if a radius is 4000 km, for a surface of 1000 km, the corners of this surface on the given sphere will be at a difference of about 31,496,588mm, that is 31.49km in front of tangent from the middle of the 1000km circle arc.
That is, 31,496,588mm (31.49 km) distance from both ends to the tangent in the middle of the circle arc.

sophiecentaur
Gold Member
We have no natural experiences of very long (flat) horizontal surfaces. The lowest horizon we can see is below a tangential line at sea level. Is it any surprise that our appreciation of ‘level’ is limited.
last night I saw some images of (parallel) ‘sunbeams’, taken with the Sun behind the camera. They appeared to converge towards a point, below the horizon. Seeing is not believing.

FactChecker
Gold Member
2018 Award
The angle off of true level in your example is very small and can only be measured with a level. The truth is that you must be significantly above sea level or the ocean waves would be at your feet. So the line toward the shore is definitely tilted down. That is what you are interpreting as "level" in the photograph, but it is not level. So the ocean appearing above that line should be no surprise.

sophiecentaur
Gold Member
Maybe you want to say that if a radius is 4000 km, for a surface of 1000 km, the corners of this surface on the given sphere will be at a difference of about 31,496,588mm, that is 31.49km in front of tangent from the middle of the 1000km circle arc.
That is, 31,496,588mm (31.49 km) distance from both ends to the tangent in the middle of the circle arc.
It's something that GPS uses all the time and the different systems use (so I believe) slightly different curvatures and centres so two ships can end up in different places with the same co ordinates. (Which can be less of a disaster than when two ships hit each other at an agreed rendezvous point when using the same GPS settings.)

DEvens