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    Re: Dip of the horizon
    From: George Huxtable
    Date: 2004 Nov 16, 21:34 +0000

    Alex wrote-
    >As I understand, the dip of the horizon is the
    >most important factor that limits the precision
    >of measuring altitudes. (I include refraction of
    >the horizon to this).
    >In the Russian navigation book I read in my youth,
    >a special device for measuring this dip was frequently
    >mentioned. It was called "naklonomer Kavraiskogo"
    >(phonetic transliteration). Kavraiskii is a name, apparently some
    >expert/inventor in navigation,
    >I see this name sometimes attached to
    >Russian devices. The word "naklonomer" has literal translation
    >I have no idea of what this device looked like, or on what
    >principle it worked. I've never seen it on e-bay, for example.
    >(First I thought that this is translated as "clinometer",
    >but then after having seen several "clinometers" on e-bay
    >I concluded that this is not so).
    >The book made an impression that measuring the dip with
    >"naklonomer" was more precise than using the dip tables.
    >The only principle I can imagine is some "bubble arrangement",
    >but I always wondered why did they have a separate device,
    >rather than the usual bubble attachment to the sextant.
    >Apparently this was something much more precise than the
    >usual bubble attachment to a sextant.
    >(I suppose the usual bubble attachment gives worse results
    >than the natural horizon under normal conditions.
    >On the other hand, the Russian book recommended to
    >"always check the dip with "naklonomer" whenever possible,
    >and to obtain high precision").
    >Did any analogous devise exist in the West?
    >How was it called then?
    >How did it work?
    Fred Hebard replied-
    Captain H.H Schufeld of the U.S. Navy published a paper in 1962 in the
    Journal of the Royal Institute of Navigation on pages 301-324 of volume
    15, entitled "Precision Celestial Navigation Experiments."  In it, he
    mentions and illustrates a Gavrisheff dipmeter.  I _guess_ that the
    instrument looks at the horizon in both directions and aligns images of
    the two horizons, measuring the angle, much like you would with a
    sextant measuring the height of church steeple.
    Searchers for that (interesting) paper (by Shufeldt, not Schufeldt) may
    find that the journal is sometimes filed under "Journal of the Institute of
    Navigation" (as it was then), and sometimes "Journal of the Royal Institute
    of Navigation" (as it is now). It's good reading for anyone interested in
    the ultimate accuracy of sextant altitudes. I have a photocopy, but it's
    not really suitable for further copying, being of a thick bound volume, so
    the text tends to disappear into the cleavage between the pages.
    Shufeldt had sextants specially made for that US Navy exercise, with
    extra-large drums, and the possibility to choose magnifications of the
    prismatic monocular, of 7x, 16x, or 20x.
    Alex questioned the spelling of Gavrisheff, whose dip-meter Shufeldt used.
    Fred has got Shufeldt's spelling of it right. It might, of course, have
    been a transcription from Cyrillic script,  which often adds ambiguity. I
    don't know if it was a Soviet instrument, or simply made in the US by
    someone with a Russian-sounding name.
    In the period of these experiments, 1959 to 1961, at the height of the cold
    war, it was unlikely to have been a recent import from Russia, but it may
    have been obtained at the time of Western-Soviet cooperation in World War
    2, in the days of the Arctic convoys. It's interesting that the most
    extreme examples of anomalous dip seem to be reported from Arctic/Antarctic
    I haven't seen any details of the Gavrisheff dipmeter, other than
    Schufeldt's description and photo. He had two, one with a telescope of 6x,
    the other (which he preferred) 20x. He says that it uses a micrometer drums
    with a vernier calibrated to 0.2', which could be interpolated to 0.1', and
    brings into view two images of the horizon, one erect and one inverted. I
    can't deduce, from the photo, how the instrument worked, but some
    listmember cleverer than I am may work it out, and if so I hope we can get
    an explanation.. Schufeldt points out that index error can be eliminated by
    taking two measurements, rotating by 180 degrees about the optic axis in
    between (a feature which I think is common to all such dipmeters.
    The oceanographic research yacht "Carnegie" included measurements of dip in
    its programme from 1904 to 1929, brought to an end by a fuel explosion
    which destroyed that unique non-magnetic vessel and killed her captain.
    Dipmeters by Zeiss were used. I've seen a company history of Zeiss which
    mentions these instruments as part of their product range, but other than
    that I know no details. However, I presume that it works as follows (or at
    least, this is how I would make one if starting again)-
    A telescope looks horizontally into a pair of angled mirrors or prisms at
    45 degrees, which give a view of the horizon to the observer's left and a
    view to his right. One mirror (or prism) can be angled slightly about the
    centre-line of the instrument, by some expanded scale which allows small
    angles to be read. The angle is adjusted to compensate for the dip, by
    aligning the two horizons.
    Then the whole thing is turned over, through 180 degrees, and the job
    repeated. The change in angle required will be 4 times the dip.
    It may be advantageous to employ the twisting of a small-angle prism, in
    one light path, to achieve the fine adjustment.
    Another dipmeter was invented by John Blish of the US Navy. This fits to a
    normal sextant. An inverted view of the horizon, behind the observer, is
    obtained when light, passing over his head enters a tall glass prism which
    acts as a back-facing periscope, and is reflected twice into the index
    mirror. The prism has to be at least as long as the vertical spacing
    between  the index mirror and the top of the observer's head, to give a
    clear view behind him. A pair of mirrors, rigidly mounted, will do the same
    job, and that's what I've used for my own version of the Blish dipmeter,
    because it's lighter and cheaper than a glass prism. It fits to my plastic
    The inverted back-horizon, seen reflected via the two normal mirrors of the
    sextant (and two extra reflections in the periscope), should coincide with
    the normal horizon seen in the horizon mirror. The index arm is adjusted
    until this happens, and a scale reading (of only a few arc-minutes) is
    noted. Then the angle is measured again, but this time with the sextant
    upside down. I hear you protest "but then it will be trying to look back
    through the observer's chest", and so it would, if the observer wasn't
    clever enough to tilt his head right over to one side, so the back-facing
    view of the horizon gets to the periscope by passing below his right ear
    (this is surprisingly easy to arrange). The scale reading, when the two
    horizons coincide, is noted again, and the difference between those two
    readings is, once again, four times the dip angle.
    It was in association with the use of the sextant for this purpose that I
    asked on Nav-l, a couple of years ago, for sextant users' experience of
    whether a sextant itself could be trusted to read the same when used either
    way up. This was the query to which Alex has recently responded. Clearly,
    if there was any flexure resulting from its own weight when inverted, then
    the periscope technique wouldn't work. The conclusion was that a sextant
    could be inverted without any noticeable change in its angular reading.
    The requirement for high precision navigation, thus calling for the
    dipmeter, was in the most demanding application of astro-navigation, the
    laying (and subsequent re-finding) of submarine cables. For most
    navigational purposes, observers would be content with a position within 3
    miles or so, but the position of a cable was needed within a fraction of a
    mile, to enable it to be dragged-for again if a fault occurred.
    In the thread "3 LOPs" Fred Hebard wrote-
    "George Huxtable's anomalous dip comes
    into play.  An LOP could be off by 10 miles because of anomalous dip
    (abnormal refraction near the horizon) and all your replications would
    not pick it up."
    This is a bit alarmist, perhaps. I think atmospheric conditions over the
    sea would have to be rather extreme, to give rise to an error of 10 miles
    due to anomalous dip. Such large errors did occur, but rarely, and
    particularly in the Arctic. But errors of 2 or even 3 arc-minutes in the
    dip were by no means uncommon. Shufeldt's paper notes that over several
    days the dip was 1 minute greater in the morning than in the evening.
    contact George Huxtable by email at george---.u-net.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.

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