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    Re: Q: how to calculate refraction at higher altitudes on land?
    From: John Kabel
    Date: 2002 Feb 28, 13:11 -0500

    Has anyone in your area sold a house or property lately?  They may have
    had a survey done.  This should have an elevation on it for a nearby
    monument.  Then you work from there with your sextant(s) and an
    artificial horizon.  Just a suggestion ...  The neighbours might be
    keen to participate in the project.
    John Kabel
    London, Ontario
    > 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@LISTSERV.WEBKAHUNA.COM]On Behalf Of Dov Kruger
    > Sent: Thursday, February 28, 2002 7:41 AM
    > 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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