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Date: Fri, 13 Nov 1998 10:07:13 -0600
From: Snowbird <snbird@ibm.net>
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Subject: Re: Estimating distance
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Logic Theorist wrote:

> Take out a sectional and mark a few landmarks that are 5 and 10 miles > away from the airports you fly to.

Unless of course you are flying to unfamiliar airports where there
don't seem to be any great landmarks at a convenient distance on
the chart (maybe there's something not charted, but you don't know)
and when you do see them well is that road really the one you 
picked out or is it a different road or....

> Some others suggested formulas or tricks by judging from runway 
> lengths, etc. These tricks don't work

I'm the only one who suggested such tricks that I've seen so far,
so I'll respond.

The way I figured out that they DO work is by being over a landmark 
of a known distance from something else and saying "now how would I 
tell how far away I was if I didn't know?".  After acquiring a GPS, I
played the same game with the GPS.  "How far away am I, using these
tricks?  OK, now how far away does the GPS say I am?"

> because as visibility increases or decreases so does your depth
> perception. 

Yes, the point is that your perception of something of known length
(the runway) will also change with altitude and visibility--so the
number of perceived runway lengths between you and the airport stays
relatively constant, being changed by the atmospheric conditions and
altitude like everything else. 

The glide path below the horizen trick depends only upon the
ability to perceive a horizen.  It is not affected by visibility,
because it depends upon principles of trig, not upon depth 
perception.

This isn't the sort of thing where one person's opinion on whether
or not it works really matters, anyone can read about it and try
for themselves and make their own decision. 

As Wolfgang Langewische points out in Stick and Rudder, there are
a number of things which "come with experience" (like the ability
to tell whether you will clear an obstacle on approach, or need
to add power) where there are actually good visual cues which will
speed up the "experience gaining" curve (like the 'spot that does
not move' trick he describes). 

But "caveat lector".  I have a sticker in my car which says "Beware
of Dingbats", my PPL CFI laughed when he saw it and said "they ought
to put that in every plane flying--then put a mirror over it.  And I
don't exempt myself".

Snowbird

