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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Marcel Tschudin
Date: 2013 Dec 29, 01:27 +0200
MarcelMay be this is of some use to you.I tried so far 3 published dip corrections, Freiesleben (1950) and Hasse (1960 and 1964). The most simple version is the one from Freiesleben, but it is also the one which agrees best with my data. I thought I would finally have been able to underst Hasse (1960) but realise now that the way how I use it may not yet be correct. This leaves at the moment only 2 versions to work with.In conditions where the air is warmer than the sea abnormal dips tend to be normal. Abnormal dips may reach negative dip values, meaning that the apparent horizon is at less than 90 degree ZD.The "abnormal" values can be estimated to a certain extend with the help of a dip correction based on the water temperature. In my dataset with over 3000 observations from the shore of the Marmara Sea 3/4 of them fall in this category. These "abnormal" conditions are expectidly less pronounced at sea (WJ Peters) because the daily temperature variations (DTR: Diurnal Temperature Range) are there minimal. The DTR and as a consequence also the chance of "abnormal" dips increases when going from the sea to coastal locations or from there even further inland.Bruce,Regarding Greg's Santa Ana observation I was in contact with him off list.
Greg described the meteorological conditions as "The air temp was 78 F / Sea temp 58 F. Wind gusting to 25 kts. from the East." He made observations with the sextant at height of eye 10ft and found that if the dip would have been instead of the +3.1' standard value a negative one of -0.4', the observation would have agreed with the GPS location (3.5' difference).
For HoE=3m and dT=11K
Freiesleben (1950):
Normal Dip: 1.82 * SQRT(3) = 3.2'
Dip correction (DECREASE) = 11 * -0.37' = -4.1'
Total Dip = -0.9' (difference 0.5')
Hasse (1964):
Normal Dip = 1.78 * SQRT(3) = 3.1'
Table 1 correction for dT and HoE = -6.0' (extrapolated)
Total Dip = -2.9' (difference 2.5')
Note that I use the authors' original factor 1.82 and 1.78 instead of 1.76 from N.A. I do this because they determined their correction relative to these values. With my dataset I obtain a very similar value than Freiesleben. It changes slightly depending whether I use 1.76 from N.A. or some other plausible value.
On Sat, Dec 28, 2013 at 7:48 PM, Bruce J. Pennino <bpennino.ce@charter.net> wrote:
Hello Greg and Everyone:Really interesting data, and because of it I did a bit of "Searching" in my theodolite measurements from the shore. I also went back and looked at the paper prepared by WJ Peters (Dip Measurements etc). First, I sometimes have trouble with the phrasing that Peters used. He said,or seems to say, " Horizon never raised more than 2.4 minutes nor depressed more than 2.0 ." I herewith quote: "Values of + 2.0 ' occur quite frequently, that is, about 1' above the average refraction at sea. The maximum depression, -2.0', occurs once in 10 years and was observed August 11,1914 in latitude 71 * N, longitude 5* W.... position on the northern edge of the Gulf Stream, and not far from the ice floes of Greenland Sea in a region where atmospheric conditions are subject to marked changes. Negative values are more rare than positive". Negative refers to a lowering of the horizon. Also, Peters states that near shore and ocean areas subject to warm breezes over cold water, etc, were not included in his data set.Looking at my 40 odd measurements, I have 7 values of clearly anomalous dip between 0.9 and 1.6 minutes of arc.These values consider the value obtained by 0.97sqrt H (ft) as normal, so my values are higher or lower by 0.9 -1.6 minutes, not much to get excited about. FIVE of these latter 7 points are looking onto Long Island Sound and Cape Cod Bay, both subject to warm winds from land or cold waters rushing in from the Race, or some odd unusual wind/water conditions. Once I fully realized the problem with these measurements, I tried to measure more dip data facing the Atlantic Ocean (but still near shore and somewhat subject to local anomalies). If I were to do additional measurements, I guess I would try to pick windy days with onshore(maybe along shore) winds. But probably depends on how much colder or warmer the air and water temperature are.On windy days less stratification?Greg, it seems you really picked a good day and site for anomalies. When the Santa Ana winds blow, you've got to find someone with a theodolite to set up with you.Very best regards,
Bruce----- Original Message -----From: Greg RudzinskiSent: Thursday, December 26, 2013 10:10 PMSubject: [NavList] In Search of Dip Anomaly
For two days now Santa Ana winds have been blowing here in Southern California. I have been waiting for these exact conditions of hot air over cold water to see if dip anomaly is present.. To check for this a series of sextant observations were made using the natural horizon which were then compared to a series of sextant observations using an artificial horizon. Intercepts using the natural horizon all fell between 3 and 4 NM from GPS. Intercepts using the artificial horizon all fell between 0 and 1 NM from GPS. It appears that 3 minutes of arc of anomalous dip is present. When prevailing conditions return then another series of observations will be made to see if the natural horizon intercepts and the artificial horizon intercepts are in better agreement.
http://en.wikipedia.org/wiki/Santa_Ana_winds
Greg Rudzinski
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