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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
From: Bill B
Date: 2005 Dec 1, 18:15 -0500
From: Bill B
Date: 2005 Dec 1, 18:15 -0500
> I'll dig out my calculations again and get back to you. Frank Still playing with your beach shots. It would appear that either your observations are approx. 4.2' minutes high or you are the victim of refraction. Since 4.2' minutes is a ton, especially for someone with your experience (and the differences in observed vs. calculated elevations are so close), I assume the Bowditch formula has problems with refraction (especially at larger distances) and/or the heat from the mills and Chicago are playing games with refraction. Following are results of my calculations using calculated angles and your observations with the Bowditch formula: ANGLES Observed Calculated Diff Sears 30.8' 26.6' 4.2' Hancock 22.1' 17.8' 4.3' ----- ----- Diff 8.7 8.8 NOTE: Refraction is not considered in calculated angles. DISTANCE (nm) Observed Calculated GPS Sears 21.07' 23.05 23.08 Hancock 21.02' 23.50 23.53 NOTE: All of these calculations assume building bases 30 ft above water level. As base height above water level increases, differences between calculated angles and observe may decease. NOTE: As Hancock is shown to be closer by observations and both fall short, I assume observed Hancock is to large. METHOD 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. Then I subtracted the hidden portion from the heights, and added 30ft for distance of base above water level. With two adjacent sides (distance converted to a chords) and an angle of the "tilt" of the buildings off the observer's plumb (determined by height of eye and distance) I used the law of cosines on the oblique triangle to establish the length of the third sides. With those established I used the law of sines to determine the angles. With the angles for Sears and Hancock established, I plugged the building heights (plus 30 feet) and the angles into the Bowditch formula. OBSERVATIONS Won't be dropping smart bombs down smokestacks with the results, but with refraction ignored, very close. Both fell slightly short. In another test I used a theoretical star at observed angles (not corrected for refraction) of 10d, 20d, and 40d with the Earth's nominal radius (converted to feet) as height of object. The diagram forms a rhombus with sides equal to the earth's radius, which seems fair game as I am not clear Bowditch addresses the foreshortening and/or backwards lean relative to the observers plumb line of tall objects at larger distances. As the altitude angle increased the distance calculated using the Bowditch formula fell further and further short of the great circle distance from observer to GP of the body. If nothing else, it supports your feeling that refraction is not adequately accounted for. Bill