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Subject: Re: Q: how to calculate refraction at higher altitudes on land?
From: Craig (c.scott@XXX.XXX)
Date: Thu Feb 28 2002 - 13:29:20 EST
If you can see the top of a nearby mountain, measure the angle above (or
below) horizontal, use the topographic map for mountain top coordinates,
waypoint mountain top and your position, obtain distance (easy with GPS),
use trigonometry for difference in opposite side, which is difference
between your height and mountain top, simple math, and voila!
-----Original Message-----
From: Navigation Mailing List
[mailto:NAVIGATION-L@XXX.XXX]On Behalf Of Dov Kruger
Sent: Thursday, February 28, 2002 13:22
To: NAVIGATION-L@XXX.XXX
Subject: Re: [NAV-L] Q: how to calculate refraction at higher altitudes
on land?
Wow, that's tough. If you don't have a horizon, then your only way of
telling your altitude with a sextant is finding the difference between
your readings of refraction and the same readings for sea level.
This won't work because the refraction really depends on air pressure,
so what you are doing is subtly using your sextant as a barometer,
something it is ill-equipped to do.. I suggest the following:
1. Take a barometer reading at your house, then at a nearby spot where
you do know the elevation, and calculate the difference. Do this on calm
days, do it a lot, and average the results. I'll bet the delta will be
fairly stable.
2. Average your GPS readings over time, and try to use readings when you
have many more than 4 satellites in view. Perhaps having a friend at a
known elevation nearby with a GPS, communicating by phone would give you
good cancellation?
3. Using a theodolite, and have a clear line of sight to something below
at a known elevation, that would work. I suppose you could try that with
a sextant as well.
4. Get a GPS with WAAS which will presumably cut the error by an order
of magnitude.
5. In theory, if you could measure the parallax of the moon with extreme
precision, you could get your height, but even without doing the math, I
can see it's impossible from a practical standpoint.
I have to say, I preferred my original conception of your problem, which
at least had a clean solution....
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