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From: Bill (no email)
Date: Mon Dec 05 2005 - 16:50:05 EST
> Adding dip back into your 30.8' figure to correct it to 0 ft height of eye,
> we have 31.1', compared to calculated (with generous refraction lift as the
> base is approx. 595 ft above sea level) of 28.6', we find your observation
> 2.5' high.
The above was a blunder. With height of eye of 11 ft, dip is 03.2'
30.8' - 3.2' dip = 27.6'
NOTE: Frank stated the horizon was 4 to 5 miles away and stated distance in
statute miles, which is SOP for Great Lakes charts. Using the mean of 4.5
sm and converting to nautical miles, dip was calculated as 3.2'. This seemed
reasonable for a beach shot. Using both sm and nm, the dip range could be
from 2.9' to 4.1'
And now the fatal flaw. Working from memory, I did not check the table
explanations, and assumed the H-h accounted for dip. Frank's 30.8' was not
adjusted for dip. IC *and* dip should be accounted for.
Below are revised results from the first of the latest group of posts:
Following are results of my calculations using calculated angles and your
observations with the Bowditch formula:
ANGLES
Frank Observed Observed - 3.2' Calculated Diff
Sears 30.8' 27.6' 26.6' 1.0'
Hancock 22.1' 18.9 17.8' 1.1'
----- -----
Diff 8.7 8.8
NOTE: As distance was known I flipped the Bowditch formula D = 1.17 sqr rt
(H-h) with height of eye as 11 ft. to establish how much of the buildings
were hidden and used the visible portion to calculate angles.
DISTANCE FROM OBSERVED & CALCULATED ANGLES (nm)
Frank Observed Calculated GPS
Sears 22.54 23.04 23.08
Hancock 22.83 23.51 23.53
====================================
In the next post the portion of the structure (Sears) below the horizon was
calculated with trig from 0 ft height of eye (471 ft). Then 153 ft lift
from refraction was added to the visible portion and the angle was
calculated at 0 ft height of eye.
ANGLE: 0d 28.6'
DISTANCE (from Bowditch formula): 22.20 nm (GPS target 23.08 nm)
I note Bowditch was written for sea-level, not Great Lakes sailors. I
understand comparing Frank's observations at approx. 595 ft above sea level
to sea-level calculations is a bit of apples-to-oranges.
Even so, I remain confused about Bowditch predictions of 389 ft hidden (sea
level) vs. trig calculated (sea level) hidden of 318 ft (471 hidden - 153
lift).
Any insights would be appreciated.
Bill
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