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Hello:

For those growing weary of dip angle minutia, I'll present some sight data for practice.

Date: April 1, 2013; DR 42* 13 N, 71 07 W; using mirror AH; actual location on Observatory Roof , Milton, Massachusetts; sun,lower limb,to the SE , IC = 0.0

Sight is interpolated from a linear best fit of 4 sights with AH: HS = 93* 15'
local time is 11:12:10 Eastern DAYLIGHT Time; watch 10 seconds SLOW. If the exact latitude of BHO is used for a time sight with the Ho, you will conclude that my sight accuracy is only fair.
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On April 1, 2013 I went to Blue Hill Observatory near Boston. It is easy to reach and has(had) a National Geodetic Survey Station (Bench Mark BM) on the top of the mountain. I was told there is a clear view to the horizon over Massachusetts Bay to the NE. The BM PID MY 3472 is covered by an 18 " concrete base for a flag pole. But good enough. I set up the theodolite on the roof, and later at a lower elevation where I used trigonometric leveling to establish a temporary bench mark , TBM. I sighted over the bay towards Logan airport; I'll present some meteorological data. In general, the day was partly sunny, cloudy, mostly cloudy at times. But, the horizon had gray sky over dark gray water; very faint horizon line. Very difficult for roof observations , somewhat better for lower sights.
All sights adjusted for MSL datum, tide level, height of roof, concrete base, etc.

Data set 4 A @ BHO height of eye 681 ft above water.
Dip angle is average of 14 sightings, front mode: dip measured = 24.5 minutes
at about 11 AM EDT ; temp 11 AM @BHO 55-60F Humidity 60% Atmos. pres 29.6" HG

Sighting toward Logan airport: Logan weather data: 11 AM 53F Humidity 64 %
12 noon 57 F, Hum 51; 2 PM 60 F hum 46%, wind at Logan was low (I think 10 mph).

Data Set 4 B @ BHO over septic field. TBM at white pipe plug
Height of eye 627 ft above water. Measured dip is a mean of seven values, looking toward NE: mean dip angle 23.1 minutes

Several lessons learned:
1. This sight actually had a limited view of horizon because of trees, islands,
hills in the distance. Best to pick a sight with at least 90* view.
2. My friend's old Garmin was useless.....elevation values in error by 15 or 20 ft, which is fine for most uses, but not this application.
3. Difficult to find Geodetic Survey station in a convenient place to see horizon.
4. All things considered, the dip equation 0.971 sqrt h is pretty good for this very elevated height of eye; difficult horizon/sky conditions.

Bruce
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