NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: dip, dip short, distance off with buildings, etc.
From: Bill B
Date: 2006 Jan 14, 00:36 -0500
From: Bill B
Date: 2006 Jan 14, 00:36 -0500
Bill, in a PS, you wrote: "Sadly IMHO, does it matter a whit when GPS is the blind King? " Frank replied: "GPS is VASTLY superior to any of this traditional navigation. It's so superior that it is hardly the same animal. But never mind that! We like traditional navigation. But if we're gonna "sell it" to other people, I think we need to do better than refer to chapter and verse in Bowditch as if it's Scripture. If someone interested in learning traditional navigation tries out Table 15 and gets a distance that's a couple of miles away from their known GPS position, it's hardly an endorsement of traditional methods! That's one reason why I think it's important to get a handle on the variability of refraction and its effect on these methods." Agreed on most points. GPS just doesn't have the "fun/skill" quotient of traditional methods for the recreational sailor looking for a challenge or backup. Back to refraction and temperature inversion, specifically the beach shots. Given: Sextant angle 30.8 minutes, height of eye 15 ft, Sears 1454 feet, GPS distance 23.08 nautical miles. Assumed: Base of Sears 30ft above water level. T15 Bowditch formula would indicate observed angle should be 0d 26.4' Using my modified constants of 0.0002001 and 0.6079 in the T15 equation, the observed angle should be 0d 28.1' 30.8' is 4.4' over stock Bowditch 30.8' is 2.7' over Bowditch with proposed values for constants Now if we divide the overshoot by 23.08 nm, this is an increase in lift per mile of 0.191' over stock Bowditch, and 0.117 over modified Bowditch constants. Going a step further: 0.15' lift-per-mile suggested by Frank for "standard" conditions. 0.157 approx value lift-per-mile used in calculating my proposed constants for T15) 0.086' lift per mile, derived from 64' lift at 20.55 miles as per Bowditch As a sanity check: 0.157 + 0.117 = .274 0.086 + 0.191 = .277 Going past three significant digits gets it closer, but nonetheless it would appear Frank's beach shots encountered approx .275' lift per mile. I should not be surprised given the range of values Frank suggested, and the heat produced by the Chicago metro area and steel-works, but I was. I am most interested to see how my SWAGs compare to Frank's thermal-inversion spreadsheet for that day. With Bowdith, it would appear one is almost always closer than the T15 table/formula would indicate. Depending on the situation, that could be good or bad. It would help if in addition to the existing refraction caution, some percentages like, "There is a 5% probability the distance will be the same as or greater than the table, an 85% probability it will be 0-7% less than the table, and a 10% probalility it will be 10-18% less than published. Great idea if the world from the poles to equator were homogenous.As a second thought, adjustment tables for refraction based on temperature and pressure (like the almanac). That falls apart for me as the use of T15 and related tables tend relate to coastal piloting (how many lighthouses or lightships are there mid Atlantic or Pacific?) Which brings us back to coasts and thermal inversions and other abnormalities. Not a rhetorical question--how should the warning label or guidelines read? Can it possibly be quantified? Also, for non coastal situations, can modifications to dip etc be adjusted by some factors? Bill