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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Distance off with Chicago buildings-corrections
From: Bill B
Date: 2005 Dec 5, 16:50 -0500
From: Bill B
Date: 2005 Dec 5, 16:50 -0500
> 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