Which Plane is Higher? Optical Illusions in Assessing Relative Altitude

Mick West

Administrator
Staff member
Which of these two planes is higher? And where are they exactly, relative to each other?



Now your first impression might be that it's fairly obviously plane B, on the right. It's visibly smaller than the left plane, so must be farther away, and its contrail is obscured by plane A the left plane's contrail, so that means the left plane is closer, right?

Wrong. The right plane is actually the one that is closer. It's actually less than half the distance away, and 4,000 feet lower.


The key here is that the aircraft are very different in size. The plane on the left , United Flight 6, is a 777-224(ER) length 209 feet, whereas the plane on the right, Alaska 682, is a 737-990, length 138 feet, or just 66% the length of the 777.

There's a variety of illusions here. For one, even though the plane on the right looks smaller, it's actually visually longer in this photo.

This matches the position and relative sizes. The right plane is the 737, 2/3 the length, but half the distance, so actually 4/3 the visual length. It looks smaller in the wider shot because the angle of the wings is more straight on, and it has a narrower body.

Another clue here's the patch of shadow on plane B's contrail.
https://www.metabunk.org/sk/20150216-152137-j8rn2.jpg
There are no other contrails or clouds in the area, so the only thing that can be leaving this shadow is plane A's contrail, so it has to be above plane B.

So why does Plane A's contrail look like it's obscuring plane B's contrail where they visually cross? That's another illusion. White contrails consist only of white light, they are not blocking any light. By our brains interpret the darker regions of the contrail as something that is blocking the light, when really it's just a hole that you can see the sky through. So it does not matter which contrail is above or below, the result is the same. This is explained in more detail here:
https://www.metabunk.org/threads/ab...ontrail-is-above-or-below-a-cloud-layer.4334/

Still not convinced? Have a look at this photo from a few seconds earlier:


Here plane B is clearly underneath plane A's contrail, even though its contrail seems to be above, proving the optical illusion.

The full set of images can be found here:
https://www.flickr.com/photos/metabunk/sets/72157648533061393/

And the Google Earth tracks of both planes are attached, which demonstrate the crossing angle in quite nicely:


A final illusion here is that the planes both appear (to some people) to be in quite steep descents. But in reality that's just perspective. The planes are both flying perfectly horizontal. The viewpoint is very close to the intersection, so the camera is looking up at it, which gives the apparent angles.

 

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Auldy

Senior Member.
Fantastic demonstration of some of the common illusions that may confuse people and lead to misunderstandings. Thanks Mick.
 

WeedWhacker

Senior Member
antastic demonstration of some of the common illusions that may confuse people and lead to misunderstandings.
YES, indeed. Fully agree (because? I "HOPE" this doesn't ruin the thread....BUT? Pilots can see contrails from "multiple" altitudes....(not just from the surface of the Earth!!).

The 'depth of perspective' matters...meaning, a contrail can be viewed from "ground level, of course....but ALSO from another airplane....at whatever altitude....so a viewing of a contrail can be "relevant" to the observer's position. WHICH (of course) relates to MANY old videos, easily found on YouTube of WW2 contrails!! So, this remains a "constant" in terms of a fact. "(about)" contrails!

CONTRAILS are real.....

"chemtrails" are NOT "real".....
 

Auldy

Senior Member.
...Pilots can see contrails from "multiple" altitudes....(not just from the surface of the Earth!!)....
Yes, I think the only way you could make this post better (however impossible) is that you had footage/photos from the cockpits of planes A and B as the trails traversed or from planes viewing the same example at varying altitudes nearby. Time to stick a goPro on the nose of every plane then? :p
 

Mick West

Administrator
Staff member
I think it's mainly because plane A's contrail is wider. That makes it look closer.
Yes, that leads to the impression that Plane A is bigger, hence closer. Looking at the side by side, there's quite a dramatic difference both in the visual spacing of the engines, and the initial size of the contrails.


The Plane A (777) contrails were also more persistent. Generally the higher plane is more likely to have more persistent contrails, but of course that's not something the average person takes into account. Instead, more persistence just makes the trail bigger, and hence it feels closer.
 

Mick West

Administrator
Staff member
People inevitable bring up the Father Ted "Small, or Far Away?" sketch in discussions of perspective. I'd advise against this, as the premise of the sketch is mocking a mentally challenged person who does not understand perspective.

In this case though, and in most cases involving planes, contrails, and perspective, the errors in in perception are perfectly understandable, and to be expected. The focus should not be on poking fun at people, but on trying to cleary explain what the illusion is, and why we all fall for it.

After all, plane A does look closer, and there's very little in the scene to suggest otherwise. The planes look like they cross quite close to each other, when they are many miles apart. We all initially misinterpret what we are seeing here.
 

WeedWhacker

Senior Member
Yes, I think the only way you could make this post better (however impossible) is that you had footage/photos from the cockpits of planes A and B
Yes, true....and can be found via numerous YouTube videos. There are a great many examples, filmed ("videoed"?) from airliner and other jet cockpits....ALL easily researched.

I will not post such a video....I think they are readily available already in the MB database history...
 

SR1419

Senior Member.
Nice job. Its something I ponder almost every time I see more than one plane and/or contrail.

Its almost impossible not to literally lose perspective when looking up at the sky. Its so hard to really tell anything about height and distance but our land lubber status tries its feeble best to coordinate...and our hubris lets us think we know.

I would love to see a time lapse cross-cut of the sky showing wind/cloud patterns, temp changes/movements, traffic etc...
 

Auldy

Senior Member.
Yes, true....and can be found via numerous YouTube videos. There are a great many examples, filmed ("videoed"?) from airliner and other jet cockpits....ALL easily researched.

I will not post such a video....I think they are readily available already in the MB database history...
Yes I know that, I was aiming more along the lines of congregating all these resources together. But that would require huge pre-planning/luck. Flightaware, Google Earth, footage/photos from the ground and then footage/photos from the actual planes or aircraft nearby. Then it would need to be the right conditions for the formation of contrails and THEN they would need to traverse... hurts my brain just imagining the logistics.
 

Hama Neggs

Senior Member.
People inevitable bring up the Father Ted "Small, or Far Away?" sketch in discussions of perspective. I'd advise against this, as the premise of the sketch is mocking a mentally challenged person who does not understand perspective.

In this case though, and in most cases involving planes, contrails, and perspective, the errors in in perception are perfectly understandable, and to be expected. The focus should not be on poking fun at people, but on trying to cleary explain what the illusion is, and why we all fall for it.

After all, plane A does look closer, and there's very little in the scene to suggest otherwise. The planes look like they cross quite close to each other, when they are many miles apart. We all initially misinterpret what we are seeing here.
I like to use the analogy of the sun appearing to orbit Earth once a day.
 

Trailspotter

Senior Member.
Yes, that leads to the impression that Plane A is bigger, hence closer. Looking at the side by side, there's quite a dramatic difference both in the visual spacing of the engines, and the initial size of the contrails.


The Plane A (777) contrails were also more persistent. Generally the higher plane is more likely to have more persistent contrails, but of course that's not something the average person takes into account. Instead, more persistence just makes the trail bigger, and hence it feels closer.
Here is a recent photo of two planes of different sizes viewed at the side by side (captured by Alan Bushell):
Interestingly, in this case the smaller A330 is actually higher at 36,000 ft than the bigger B747 flying at 30,000 ft. The A330 length of 63.7 m is about 90% of the B747 length of 70.6 m, but it looks extra 10% smaller in the photo being farther away from the camera.
 

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JFDee

Senior Member.
This is annother exhibit that I came across on FB. Unfortunately, I couldn't find the original source yet.

close_or_not.jpg

My take is that the plane at 'the front' is a Boeing 737-300 or something in that class, and the one 'behind' is a Boeing 767-400 or similar - which is much larger in comparison and therefore must be considerably higher than the other plane.

It also looks like the second plane is creating the trail.
 

Trailspotter

Senior Member.
This is annother exhibit that I came across on FB. Unfortunately, I couldn't find the original source yet.

View attachment 11820

My take is that the plane at 'the front' is a Boeing 737-300 or something in that class, and the one 'behind' is a Boeing 767-400 or similar - which is much larger in comparison and therefore must be considerably higher than the other plane.

It also looks like the second plane is creating the trail.
I remember seeing it at Chemtrails Project UK photos. There was a series, starting with this:
https://www.facebook.com/Chemtrails...0.1426083165./814002031967455/?type=3&theater
There is also a picture in the comment to the above link showing the two planes aligned at practically the same position in the sky:

I think that the original source of these pictures was a video. I have a vague recollection of seeing it on flickr, but I can't find the link at the present.
 

Spectrar Ghost

Senior Member.
This is annother exhibit that I came across on FB. Unfortunately, I couldn't find the original source yet.

View attachment 11820

My take is that the plane at 'the front' is a Boeing 737-300 or something in that class, and the one 'behind' is a Boeing 767-400 or similar - which is much larger in comparison and therefore must be considerably higher than the other plane.

It also looks like the second plane is creating the trail.
The front plane looks like an A320 variant to me, but that doesn't really change your conclusions.
 

Mick West

Administrator
Staff member
I remember seeing it at Chemtrails Project UK photos. There was a series, starting with this:
https://www.facebook.com/Chemtrails...0.1426083165./814002031967455/?type=3&theater
There is also a picture in the comment to the above link showing the two planes aligned at practically the same position in the sky:

I think that the original source of these pictures was a video. I have a vague recollection of seeing it on flickr, but I can't find the link at the present.
There's a full thread on these photos:
https://www.metabunk.org/threads/two-planes-at-the-same-altitude-only-one-leaves-a-trail.4358/
 

WeedWhacker

Senior Member
Worth noting that there can also be a lot of variation within a single plane type, like the 777
THAT (^^^) is an important...a very important point. This is something that people familiar with aviation understand almost intuitively.

Unfortunately, many who are so-called 'amateur' plane-spotters? Might not be fully well-aware of these subtle variations, and this can lead to misconceptions of course.
 
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