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Subject: Re: Q: how to calculate refraction at higher altitudes on land?
From: Dan Allen (danallen46@XXX.XXX)
Date: Thu Feb 28 2002 - 13:03:39 EST
I am specifically trying to determine the elevation of my house.
The topographic maps of my area are decades old. They do not show
our streets or houses, and in fact the area has been graded to some
extent as well, so even finding my exact location via GPS and then
looking on the map only gives a ballpark figure.
Since SA has been turned off, the GPS gives a range of elevations
from 580 feet to 650 feet. The Garmin GPS 3 tends to wander over
this range. I am trying to get a better value.
I have a high quality Chelsea barometer, a Garmin GPS, and many
sextants. I have high mountains behind my house, lots of clouds,
and the only kind of horizon I'll ever see is an artificial one.
In the winter, the sun comes up behind the mountain at about 11AM
and goes down about 12:30PM!
Given these constraits and tools, can I get a better estimate of
my elevation?
-----Original Message-----
From: Navigation Mailing List
[mailto:NAVIGATION-L@XXX.XXX]On Behalf Of Dov Kruger
Sent: Thursday, February 28, 2002 7:41 AM
To: NAVIGATION-L@XXX.XXX
Subject: Re: Q: how to calculate refraction at higher altitudes on land?
Oops!
Dan,
At high angles (above 45 or so) you will have so little refraction in
the first place that any reduction in it won't make a significant
difference with what I originally said. You can just do your atmospheric
correction and that's more than it deserves. But at the lower angles,
the normal correction for pressure will presumably be too great, because
the reason the pressure is low is that you are high, not because your
whole region is experiencing low pressure.
In the worst case, consider you are looking down at the horizon. Near
the horizon, your line of sight is passing through sea-level air.
Halfway, it is passing through air at half your altitude. Since the
correction is small in any case, why not just try to divide it in half
and use that? You know the upper bound (no pressure correction) and the
lower bound (full pressure correction) so you know exactly how bad your
assumption can be.
Why not send us the raw data when you do it so we can take a look?
cheers,
Dov
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