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
----- Original Message -----
Sent: Friday, December 27, 2013 7:34
PM
Subject: [NavList] Re: In Search of Dip
Anomaly
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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