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Hello:

 

I wanted to get this data out for all to see.  The "wild goose" chase herewith ends for me. It has been interesting and I've learned a lot. Attached are two tables related  to dip: Table 1 shows the actual dip data I've recorded over the past 10 months; Table 2 (HO-HC) shows my method  of evaluating the overall accuracy of the measurements. The two photos show the system of measuring the dip data.  The surveying equipment is commonly called a theodolite, but it is normally called a total station by surveyors.  It is old by modern standards, but the optics are great and it can measure angles and distance. It is a 6 second "gun", but easily can read to 3 seconds of arc , and maybe estimate to the nearest two seconds. Table 1 includes meteorological conditions.

 

Using all of the data recorded from shore, some sighting Long Island Sound but mostly sighting the horizon on Cape Cod Bay or an Atlantic Horizon, the dip coefficient was calculated to be 0.87, compared to 0.97 in the NA.  There is considerable variation with the air and water temperature (and wind).  Note that most of the data were recorded with the air temperature at  or above the water temperature.  As Marcel recently posted, this air/water condition often produces a smaller  actual dip angle than shown in NA.

 

On some of the test dates, the theodolite was used to measure a zenith angle to obtain a LOP.  Table 2 shows  (HO - HC) calculated from knowing my exact location.  Most of the dip angles are an average of 10 sighted values over a period of 4- 5 minutes of time. Collimation errors etc were all small, 1-2 seconds or so of arc. The single biggest potential source of error was estimating wave height with the prism pole at the edge of the surf or in the water on very calm days. Data were not recorded on very rough surf days, especially  at small values of "height of eye".  In general, the height of eye is precise to probably +/- a few percent, but depends on many factors for a specific data point.  On some points I surveyed in bench marks. Other points I used  elevation  surveys done by USGS or professional land surveyors. In all cases I confirmed the heights in several different ways (usually by direct measurement). As an extreme example, there was no easy way to check that the  top of the  Blue Hill Observatory was 676 ft  above the water level at the time of the sight.  If there was doubt, the data was redone or thrown out.  If my results seemed unusual, I went back to the site several times.  Notice the early tests done at Avery Point (AP) with a view to The Race in one direction and then down Long Island Sound at 240 degrees magnetic (same setup without moving the theodolite). 

 

For those taking sextant sights in coastal areas, particularly when the air is warmer than the water, the dip on average is probably 8-10% less than indicated by the NA . For a HoE of 25 ft or less, the dip error might be about

0.5 - 1 minute of arc. Using the sighted dip data , my largest difference with the NA was about 1.6 minutes (in one case larger and in one case smaller).  Typical differences were a minute or so , on average.

 

For those on docks or very stable vessels with closely measured height of eye, it would be interesting to know if LOPs or (HO-HC) improve if a smaller value of dip is used in sight reduction. I suggest  0.89sqrt HoE ft  (WJPeters).  Or, if you have several excellent sets of data, does ( HO - HC) improve slightly with a smaller dip?

 

Lastly, if someone wants to get more data, I suggest moderately windy days (reduced stratification and hopefully more like an offshore condition), HoE at least 12 or 15 ft to minimize relative errors in the measurement of height, and days with air temperature colder than water temperature by at least 5 F ( might show very unusual dip). 

 

Beavertail Point (lighthouse) is an excellent location, but I learned of it too late in the data acquisition.  It has a good high view of the colder Atlantic water horizon,  can easily climb down to the water for an accurate elevation, good parking, close to coffee shop or place for refreshment. Lots of tidal and weather data available. Test No 28 at Beavertail showed a KM = 0.96 and FER was a witness. Air and water temp were 64 and 65 F, respectively. 

 

If anyone wants to discuss this further, feel free to contact me off-list, or of course onlist if of general interest.  

 

Best regards to all and hope that  2014 has begun on a happy note!

Bruce

 

Phone : (508) 829 - 7131

Email: bpennino.ce---.net

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

----- Original Message -----

From: Greg Rudzinski

To: bpennino.ce---.net

Sent: Friday, December 27, 2013 7:34 PM

Subject: [NavList] Re: In Search of Dip Anomaly

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Prevailing weather conditions have moved in today here at Oxnard, California. A second series of observations has been done at the beach to compare the artificial horizon intercepts with natural horizon intercepts. The artificial horizon set of observations reduced to an average intercept of 0.1' toward with a spread of +/- 0.3'. The natural horizon set of observations reduced to an average intercept of 0.5' toward with a spread of +/- 0.6'. This is good agreement between the two sets and confirms the Santa Ana wind dip anomaly effect observed in yesterdays observations..

Greg Rudzinski

In Search of Dip Anomaly
From: Greg Rudzinski
Date: 2013 Dec 26, 19:06 -0800
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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