I'd never seen this before

On Christmas Eve, I got to go flying. I took a friend of ours on a 200 nm
cross country to drop her off with her family for the holidays. On the good
side, the visibility was as good as I've ever seen in the Southeast.
Probably 150 miles or more. I could see the Smokey Mtns waay north. Also,
on the way "out", there was a 25 knot tailwind, so that portion of the trip
went pretty quickly.

On the way home, I flew down low (3000 -3500' MSL) to duck most of the wind,
but still enjoyed the good visibility. Part of the way home, I saw a large
tower a long way in front of me. I was surprised, because the tower
appeared to be much higher than my cruising altitude. I didn't remember any
extremely tall towers NE of Atlanta, but studied the sectional several times
looking for what my eyes were seeing. Never did find a particularly tall
tower on the sectional.

Anyway, it probably took me 20 minutes to reach the tower and by the time I
reached it, it was well below my altitude, (which hadn't changed). After a
little thought, I realized that the curvature of the earth had resulted in
an illusion that the tower was extremely tall when viewed from a distance,
but was only 1000' AGl (or 1800' MSL) in reality.

I've heard of this phenomena causing confusion between aircraft approaching
each other at high altitude, but never realized it would occur with towers
and the like when visibility was good. It'll probably be a long time
before the visibility around here is good enough to see a tower from 50
miles away again.

"Kyle Boatright" wrote in
:

On Christmas Eve, I got to go flying. I took a friend of ours on a
200 nm cross country to drop her off with her family for the holidays.
On the good side, the visibility was as good as I've ever seen in the
Southeast. Probably 150 miles or more. I could see the Smokey Mtns
waay north. Also, on the way "out", there was a 25 knot tailwind, so
that portion of the trip went pretty quickly.

On the way home, I flew down low (3000 -3500' MSL) to duck most of the
wind, but still enjoyed the good visibility. Part of the way home, I
saw a large tower a long way in front of me. I was surprised, because
the tower appeared to be much higher than my cruising altitude. I
didn't remember any extremely tall towers NE of Atlanta, but studied
the sectional several times looking for what my eyes were seeing.
Never did find a particularly tall tower on the sectional.

Anyway, it probably took me 20 minutes to reach the tower and by the
time I reached it, it was well below my altitude, (which hadn't
changed). After a little thought, I realized that the curvature of
the earth had resulted in an illusion that the tower was extremely
tall when viewed from a distance, but was only 1000' AGl (or 1800'
MSL) in reality.

I've heard of this phenomena causing confusion between aircraft
approaching each other at high altitude, but never realized it would
occur with towers and the like when visibility was good. It'll
probably be a long time before the visibility around here is good
enough to see a tower from 50 miles away again.


Well, even at middling altitudes I find it hard to tell the altitude of
other airplanes. I also find it difficult to tell the height of a tower,
even relative to my own altitude. if there's a trick to it aside from
makrking the windscreen with a magic marker, it's eluded me!
The introduction of TCAS was a revelation. NEarby airplkanes that you
might have sron were a couple of thousand above turned out to be lower
than us and vice versa. It's not just me, everyone says the same thing.
I can't see how the curvature of the earth would do it, though. Seems to
me no matter which way you slice that the tower would look lower.




Bertie

That part of the country is pretty flat, so something a thousand feet
agl might look really high from a distance. From your altitude, did it
appear higher than the Smokies? That would do a number on your
perception.




On Jan 1, 8:27*am, Bertie the Bunyip wrote:
"Kyle Boatright" wrote m:





On Christmas Eve, I got to go flying. *I took a friend of ours on a
200 nm cross country to drop her off with her family for the holidays.
*On the good side, the visibility was as good as I've ever seen in the
Southeast. Probably 150 miles or more. I could see the Smokey Mtns
waay north. * Also, on the way "out", there was a 25 knot tailwind, so
that portion of the trip went pretty quickly.

On the way home, I flew down low (3000 -3500' MSL) to duck most of the
wind, but still enjoyed the good visibility. *Part of the way home, I
saw a large tower a long way in front of me. *I was surprised, because
the tower appeared to be much higher than my cruising altitude. *I
didn't remember any extremely tall towers NE of Atlanta, but studied
the sectional several times looking for what my eyes were seeing.
Never did find a particularly tall tower on the sectional.

Anyway, it probably took me 20 minutes to reach the tower and by the
time I reached it, it was well below my altitude, (which hadn't
changed). * After a little thought, I realized that the curvature of
the earth had resulted in an illusion that the tower was extremely
tall when viewed from a distance, but was only 1000' AGl (or 1800'
MSL) in reality.

I've heard of this phenomena causing confusion between aircraft
approaching each other at high altitude, but never realized it would
occur with towers and the like when visibility was good. * It'll
probably be a long time before the visibility around here is good
enough to see a tower from 50 miles away again.

Well, even at *middling altitudes I find it hard to tell the altitude of
other airplanes. I also find it difficult to tell the height of a tower,
even relative to my own altitude. if there's a trick to it aside from
makrking the windscreen with a magic marker, it's eluded me!
The introduction of TCAS was a revelation. NEarby airplkanes that you
might have sron were a couple of thousand above turned out to be lower
than us and vice versa. It's not just me, everyone says the same thing.
I can't see how the curvature of the earth would do it, though. Seems to
me no matter which way you slice that the tower would look lower.

Bertie- Hide quoted text -

- Show quoted text -

"Tina" wrote in message
...
That part of the country is pretty flat, so something a thousand feet
agl might look really high from a distance. From your altitude, did it
appear higher than the Smokies? That would do a number on your
perception.

No, it did not look higher than the Smokies, which were well off to my right
on the return trip. It did look higher (and probably was) than any of the
hills within about 50 miles of Atlanta.




On Jan 1, 8:27 am, Bertie the Bunyip wrote:
"Kyle Boatright" wrote
m:





On Christmas Eve, I got to go flying. I took a friend of ours on a
200 nm cross country to drop her off with her family for the holidays.
On the good side, the visibility was as good as I've ever seen in the
Southeast. Probably 150 miles or more. I could see the Smokey Mtns
waay north. Also, on the way "out", there was a 25 knot tailwind, so
that portion of the trip went pretty quickly.

On the way home, I flew down low (3000 -3500' MSL) to duck most of the
wind, but still enjoyed the good visibility. Part of the way home, I
saw a large tower a long way in front of me. I was surprised, because
the tower appeared to be much higher than my cruising altitude. I
didn't remember any extremely tall towers NE of Atlanta, but studied
the sectional several times looking for what my eyes were seeing.
Never did find a particularly tall tower on the sectional.

Anyway, it probably took me 20 minutes to reach the tower and by the
time I reached it, it was well below my altitude, (which hadn't
changed). After a little thought, I realized that the curvature of
the earth had resulted in an illusion that the tower was extremely
tall when viewed from a distance, but was only 1000' AGl (or 1800'
MSL) in reality.

I've heard of this phenomena causing confusion between aircraft
approaching each other at high altitude, but never realized it would
occur with towers and the like when visibility was good. It'll
probably be a long time before the visibility around here is good
enough to see a tower from 50 miles away again.

Well, even at middling altitudes I find it hard to tell the altitude of
other airplanes. I also find it difficult to tell the height of a tower,
even relative to my own altitude. if there's a trick to it aside from
makrking the windscreen with a magic marker, it's eluded me!
The introduction of TCAS was a revelation. NEarby airplkanes that you
might have sron were a couple of thousand above turned out to be lower
than us and vice versa. It's not just me, everyone says the same thing.
I can't see how the curvature of the earth would do it, though. Seems to
me no matter which way you slice that the tower would look lower.

Bertie- Hide quoted text -

- Show quoted text -

Hi,

In article ,
Bertie the wrote:
Well, even at middling altitudes I find it hard to tell the altitude of
other airplanes.

I read somewhere that if the other plane appears above the horizon, it's
above you. Similarly, if it appears below the horizon, it's below you.

Obviously this will change depending on whether you're climbing or
descending, but as a general rule it sounds like it could make sense.

Haven't really had the chance to test it myself since I read it though.

Andy

Andy Hawkins wrote in
:

Hi,

In article ,
Bertie the wrote:
Well, even at middling altitudes I find it hard to tell the altitude
of other airplanes.

I read somewhere that if the other plane appears above the horizon,
it's above you. Similarly, if it appears below the horizon, it's below
you.

Obviously this will change depending on whether you're climbing or
descending, but as a general rule it sounds like it could make sense.

Haven't really had the chance to test it myself since I read it
though.


Well, we often can't see the horizon, suppose that;s the difficulty. It's
easier low down, though. Lots easier.

Bertie

On Jan 3, 6:41 am, Andy Hawkins wrote:
Hi,

In article ,
Bertie the wrote:

Well, even at middling altitudes I find it hard to tell the altitude of
other airplanes.

I read somewhere that if the other plane appears above the horizon, it's
above you. Similarly, if it appears below the horizon, it's below you.

Obviously this will change depending on whether you're climbing or
descending, but as a general rule it sounds like it could make sense.

Haven't really had the chance to test it myself since I read it though.


I stopped at putting the finger on the wind shield over the other
aircraft.
If it moves from under my finger I keep on course.
If it doesn't appear I change course, altitude or airspeed until it
does appear.
The horizon trick seems valid though I've never tried (or even thought
of it until now)
Thanks

Kyle Boatright writes:

Anyway, it probably took me 20 minutes to reach the tower and by the time I
reached it, it was well below my altitude, (which hadn't changed). After a
little thought, I realized that the curvature of the earth had resulted in
an illusion that the tower was extremely tall when viewed from a distance,
but was only 1000' AGl (or 1800' MSL) in reality.

The curvature of the planet won't do this; it makes things seem lower, not
higher, just as a tower behind a hill might not appear as tall as it does once
you reach the crest of the hill.

However, some atmospheric effects can make things seem larger or taller than
they are from a distance.

At an altitude of 3000 feet AGL over smooth terrain, you'll be able to see the
top of a 1000' tower (but not the whole thing) from up to 92 nm away.

On Tue, 01 Jan 2008 18:19:29 +0100, Mxsmanic wrote:

Kyle Boatright writes:

Anyway, it probably took me 20 minutes to reach the tower and by the time I
reached it, it was well below my altitude, (which hadn't changed). After a
little thought, I realized that the curvature of the earth had resulted in
an illusion that the tower was extremely tall when viewed from a distance,
but was only 1000' AGl (or 1800' MSL) in reality.

The curvature of the planet won't do this; it makes things seem lower, not
higher, just as a tower behind a hill might not appear as tall as it does once
you reach the crest of the hill.

No. Pilots are accustomed to judging the altitude of other aircraft based on
whether it's above or below the horizon. This is great for collision avoidance,
but doesn't work at longer ranges. ANYTHING located at the viewer's horizon
will appear above the altitude of the observer, and it *is* due to the curvature
of the Earth. See:

http://www.wanttaja.com/los.jpg

Normally, though, details at such distances are obscured by haze.

Ron Wanttaja

Ron Wanttaja writes:

No. Pilots are accustomed to judging the altitude of other aircraft based on
whether it's above or below the horizon. This is great for collision avoidance,
but doesn't work at longer ranges. ANYTHING located at the viewer's horizon
will appear above the altitude of the observer, and it *is* due to the curvature
of the Earth. See:

http://www.wanttaja.com/los.jpg

Normally, though, details at such distances are obscured by haze.

But the original post made no mention of the horizon.

On a flat surface of infinite extent, the horizon is always at eye level, no
matter what your position. On a flat surface of finite extent, it is always
slightly below eye level, depending on how far away the edge of the surface
is. On a spherical planet, the horizon is still lower; to find its distance
(assuming smooth terrain), add your altitude to the radius of the planet,
square it, subtract the radius of the planet squared, and take the square root
of the result. At an altitude of 3000 feet above smooth terrain (such as
water, or a dry lake), the horizon on Earth is 58 nm away. If you are six
feet tall and standing on the surface with smooth terrain (or if you are in a
rowboat on a calm ocean, for example), the horizon is just 2.6 nm distant.

Your diagram is interesting, but since it dramatically overstates the height
of towers and dramatically understates the size of the planet, it's a bit
misleading. Your towers would be several times higher than the orbit of the
International Space Station, and the aircraft would be in outer space.

On a flat surface, anything moving down in your field of view is something
you'll fly over, and anything moving up is something you'll hit. The
curvature of the Earth complicates this, but the curvature is gentle enough
that anything affected by it is too far away to be an immediate hazard,
anyway. At 3000' AGL, you could see Mount Everest from 239 nm away, but since
a small plane might take two hours reach it, you'd have plenty of time to
evaluate it as a hazard.

Thought experiments like this can be interesting. People often say that the
Concorde was wonderful because you could see the curvature of the Earth, but
the fact is that you can see the curvature from anywhere, even a hill
overlooking the beach. It just gets more obvious as you move further away
from the surface. From an airliner at 39,000 feet, the view extends for well
over 210 miles in every direction. I was once amused to discover on a flight
from Phoenix to Los Angeles that I could see both cities from my window at the
same time at the midpoint of the flight.