A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Sextant Accuracy and anomalous dip.
From: Arthur Pearson
Date: 2003 Mar 17, 22:53 -0500
From: Arthur Pearson
Date: 2003 Mar 17, 22:53 -0500
Gentlemen, George refers to anomalous dip that occurs "when the air-layers within a few feet of the horizon differ in temperature in an unexpected way, and refract accordingly." I stumbled into this phenomenon two years ago comparing sights taken in Maine and the Caribbean. I was taking multiple sun sights whenever I could get a clear horizon during our summer trips among the Maine islands. Compared to an "assumed position" from GPS, my sights most frequently indicated an intercept 2-4 nm "away", which suggested I was under-measuring the altitude by 2' to 4'. I thought it was my technique or instrument error until I repeated the same exercise during a week long cruise in the Caribbean. Those sights were typically +- 2' either side of truth, showing none of the same bias toward consistent under-measurement I had experienced in Maine. I found one possible explanation on pages 424-425 of my 1958 Bowditch where Sea-air temperature difference correction (S) is discussed in the chapter on Sextant Altitude Corrections. Bowditch explains that standard dip tables assume a standard air density gradient from sea level up into the sky. That standard gradient is effected by many factors, including the relative sea-air temperature. Bowditch states: "If the water is warmer than the air, the horizon is depressed and dip is increased. Under these conditions the measured altitudes are too great. Therefore, as a correction to the altitude, the sea-air temperature difference correction is negative when the water is warmer than the air. When the air is warmer, the reverse is true, and the altitude correction is positive." Most of my Maine sights were on warm days (75-80�F) on the cold waters Gulf of Maine (55-60�F), a differential of in the range of 20-25�F. Bowditch indicates this would result in the measured altitudes being too small which is exactly what I experienced in the consistent bias of my Maine sights. This theory is supported by my Caribbean sights which displayed no such bias under conditions where sea and air temperatures were within 10�F of each other. Bowditch also states that "The effect is greater as the temperature difference increases." He notes that studies attempting to derive a correction factor based on the sea-air temperature difference "differ considerably". However, he summarizes the range of results and reports "the average of these [studies] is about 0.16' per degree Fahrenheit (0.28' per degree Celsius). Thus, the correction is about one-sixth of a minute per degree Fahrenheit, or one minute per each six degrees." This rough rule of thumb when applied to my Maine sights (temperature difference of 30�F) would suggest a correction of about 3' to 4' which is strikingly consistent with the 2' to 4' bias I experienced in my Maine sights. While I won't be measuring sea and air temperature before my next round of sights, I now keenly aware the potential impact of non-standard atmospheric conditions on sight accuracy. Regards, Arthur -----Original Message----- From: Navigation Mailing List [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM] On Behalf Of George Huxtable Sent: Monday, March 17, 2003 2:53 PM To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM Subject: Re: Sextant Accuracy and anomalous dip. Fred Hebard wrote- >The location of my house seems to be settling down to perhaps 0.3 to >0.4 nautical miles north of its actual location. All of this with an >artificial horizon. One new technique I tried was to preset the >sextant to the future altitude of a body and then watch the body pass >through that altitude while listening to the time being counted out. > >My general objective is to get these observations as accurate as >possible, say to within 0.1 or 0.2 miles. I'm not sure why I have this >objective, but it persists. > >Bill Murdoch mentioned he had never bothered to apply instrument >corrections to readings taken with his Freiberger, implying that it's >not necessary to achieve accuracies better than about 1 nautical mile. >That also seems to be the general opinion here, especially with regard >to observations from small yachts, where it often is not possible to >get more accurate. But how about larger vessels? Does anybody want to >defend trying to get accuracies closer than 1 mile? Comments from George. Fred has every right to be satisfied with these results. Presumably he has rather sharp eyesight. It would be interesting to know how much scatter there is between Fred's individual measurements of the position of his house. Is his result of "perhaps 0.3 to 0.4 miles Morth" an average of many observations, perhaps? I'm interested in how closely they cluster about that value. I am presuming that the artificial horizon used is some form of reflecting pool. Altitude measurements made that way on land have several advantages over horizon altitudes measured from a boat. Because the doubled altitude is measured on the sextant, the effect of sextant calibration (and other) errors is halved. With a stable platform underfoot, the observer isn't being buffeted and his images are steady, so he can use a high-gain telescope. If a glass cloche is being used as a windbreak, care has to be taken over the optical quality of the glass panes used. The main errors in measuring an altitude at sea arise from deficiencies in the sea-horizon that has to be used as a reference: deficiencies that are absent in artificial-horizon observations on land. From a small boat at sea, the horizon is not a steady straight line but a succession of overlapping wavecrests, observed from a platform that's also heaving up and down, changing the perspective view of the waves and altering the dip from moment to moment. A big vessel, with its slow and predictable roll, and its high viewpoint, is MUCH less affected in that way. Even in millpond conditions, however, when the small boat loses much of that disadvantage, there remains a source of error affecting the horizontality of the horizon, for small vessels and large ones alike. This is an old "hobby-horse" of mine, and I hope nobody minds if I trot it out for a ride once again. This error is "anomalous dip", when the air-layers within a few feet of the horizon differ in temperature in an unexpected way, and refract accordingly. It causes dip to vary slightly, in an unpredictable way, and it's hard to detect it happening, let alone estimate how much to allow for. This effect has nothing to do with the small corrections for refraction, depending on temperature and pressure, which most almanacs provide. Changes of dip of a minute or so from the predicted value can be quite frequent, changes of 2 or 3 minutes can occasionally happen, changes of 5 minutes are possible though very unusual. These can perturb a sextant altitude by a corresponding amount, and the observer will be completely unaware that it has happened. So if you are a sextant-navigator wishing to avoid a dangerous hazard, how much room do you allow to consider your vessel safe? Usually the dip-error will be within a minute, but it's hardly satisfactory to say that "usually" you will miss that dangerous rock. You wouldn't wish to navigate in such a way as to hit it "only occasionally". A safety-standard of "almost never" is I think the very minimum anyone would accept, and any prudent navigator would insist on a better margin even than that. So a navigator without knowledge of the dip-of-the-moment might be wise to allow a margin of 5 miles or so, to cater for possible anomalous-dip. Dipmeters have been invented and used in oceanographic survey work. A crude but effective dipmeter can be knocked together without great expense. It requires a view of the horizon in two opposite directions to be visible simultaneously from somewhere on deck. There may be a case for a quick check of the dip as a matter of routine whenever sextant observations are made. The results would probably be very boring. Most of the time the dip-error would be well within one arc-minute. But on those rare occasions when it was considerably greater, the navigator would be forewarned, and could act to correct it. That correction would allow him a greater reliance on the precision of his sextant observations. Anomalous dip seems to be generally disregarded as a hazard to sextant navigation. However, I think it's worth taking seriously. There's more stuff about anomalous dip in old archives of this mailing list. George Huxtable ================================================================ contact George Huxtable by email at email@example.com, by phone at 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ================================================================