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    Re: Very Small Altitude-Accurate Results
    From: Frank Reed
    Date: 2015 Aug 10, 13:11 -0700

    Sources that recommend shooting altitudes only above 10° may be doing so for two independent reasons:

    • because they assume you don't have a temperature/pressue correction table handy,
    • or they be following an ancient bit of lore that hasn't been valid in decades.

    Temperature and pressure corrections are directly proportional to the mean refraction. At low altitudes, primarily below five degrees, these can get large enough to worry about even under apparently "normal" weather conditions. Bruce, in your sights yesterday evening, did you do any correction for temperature and pressure? The weather has been very nice this weekend in New England, and generally "nice" weather corresponds with "normal refraction" weather, so maybe it was just luck of the draw. You may want to compare the results with and without the T/P corrections. In general, if you don't have access to temperature and pressure data or the necessary corrections, you may want to stick with altitudes above 5 or 10°. 

    Many older navigation manuals and also relatively recent navigation manuals written with an emphasis on navigation "lore" will tell you that refraction tables can't be trusted below ten degrees altitude, even if you do correct for temperature and pressure. This isn't true, and it hasn't been true since the middle of the 20th century, at least. So long as you're careful about temperature and pressure (and also altitude above sea level, for aviation use and land-based sights from high locations), sights below ten degrees and even fivee degrees are just as good as sights at higher altitudes. There's no reason to avoid them if you don't mind the small amount of extra work for the T/P corrections. Of course, there is a lower limit to this.

    At extremely low altitudes, less than a degree, refraction tables become undependable due to the so-called "terrestrial refraction". This is the aspect of refraction that depends on the layering of temperature in the lowest few meters of the atmosphere. It's unpredictable. This is the "anomalous" refraction that we have discussed before. Myself, I consider sights below 1.5° questionable. This is all a matter of "degree", pardon the pun. Even at one degree altitude, the variability from anomalous refraction apparently rarely exceeds 3 minutes of arc. In many cases this might be considered nothing to worry about. Even right at the horizon, when the Sun's LL just touches the horizon, a sight still has value (and can be taken without a sextant!), but here I would consider a variation of 3 minutes of arc as normal, and a variation of 5 minutes of arc would not be rare. In any case, you must include temperature and pressure corrections at such low altitudes.

    Frank Reed
    Conanicut Island USA


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