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Bowditch Table 15
From: Trevor Kenchington
Date: 2005 Jan 23, 00:36 -0400
From: Trevor Kenchington
Date: 2005 Jan 23, 00:36 -0400
I am gradually working my way through Bowditch (1995 edition), trying to be sure that I understand everything that is relevant to smaller vessels that the book is really intended for. Most I can follow, sometimes with a bit of effort, but I am having trouble with Table 15. Since the table numbers have changed between some editions, Table 15 in 1995 was the one providing "Distance by Vertical Angle Measured Between Sea Horizon and Top of Object Beyond Sea Horizon". (That was Table 9 in at least one edition of Bowditch.) The text description accompanying the table says that it provides the distance to the object (by implication: from the observer to the object), given a knowledge of the difference in height between the observer's eye and the object, plus a vertical angle measured between the top of the object and the horizon (that angle being corrected for IE and dip). [The Bowditch main text doesn't seem to refer to Table 15 and mixes this business of an object beyond the horizon with the common business of distance off by vertical angle of an object of known height whose waterline is visible.] Table 15 itself does give distances from observer #1 to the object observed, if the observer #1 has zero height of eye and the top of the object is just dipping below the horizon. At least, the tabulated distance then equals the horizon distance for an observer (#2) at the top of the object, which should be the same thing. However, if Table 15 was to be believed, as observer #1 climbs the mast of his boat and increases his height of eye, the object observed would drop below his horizon (assuming its true distance did not change), which is obviously nonsensical. So ... if Table 15 does not give the distance from observer to object, except in special cases, what does it provide? I'm guessing that it might be the distance from the object to the observer's horizon, to which the observer's horizon distance must be added to get the full distance from observer to object. However, I do not know of any way to either confirm of refute that idea (save for a full-scale experiment, which is a bit beyond my resources just now). Can anyone sort this out for me? Trevor Kenchington -- Trevor J. Kenchington PhD Gadus@iStar.ca Gadus Associates, Office(902) 889-9250 R.R.#1, Musquodoboit Harbour, Fax (902) 889-9251 Nova Scotia B0J 2L0, CANADA Home (902) 889-3555 Science Serving the Fisheries http://home.istar.ca/~gadus