NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Taking four stars for checking accuracy of fix
From: Hewitt Schlereth
Date: 2008 Aug 1, 23:38 -0400
From: Hewitt Schlereth
Date: 2008 Aug 1, 23:38 -0400
I'm new here and your reference to NavList archives caught my eye. How does one access them? HewS On 8/1/08, Peter Foggwrote: > > Greg wrote: > > A quick google search of TITANIC web sights showed the TITANIC's > > ocean floor position to be in good agreement with the distress call > > latitude but not the longitude where there is a 20 minute discrepancy. > > What would explain this discrepancy best chronometer error or DR speed > > error? The water was very cold at the time so I doubt that the gulf > > stream was a factor. > > > 1. Sights taken "before 8 o'clock", collision "at 11.43, I think" > during which interval the Titanic was steaming full bore to the west. > Sinking quite some time after the collision, during which time the > ship was drifting ... back to the east (it was a calm night), although > at a considerably reduced rate. > > 2. The sea bottom was deep below the water surface, and that long way > down taken by the ship may not have been vertical ... > > As to the elimination of constant error, this was discussed in some > detail, complete with diagrams, some time ago (thus findable in our > archives). > > Yes, sights using opposing azimuths will lead (with constant error > only present, ie; assuming no erratic error which could complicate > things) to a 'box' shape. Since with each pair of intersecting LOPs > the true fix will lie along a line bisecting the angle formed by their > intersection, with such a box shape the fix will be found ... at the > centre of that shape! > > How about that. Who would have thought. Incidentally, the same also > holds true for a triangle formed by 3 LOPs. Assuming only a constant > error, and a spread of azimuths of more than 180d, the fix (free of > that constant error) must be found at the centre of that triangle. > > It is only when the assumption is made that the sights are somehow > free of any constant error, and thus subject only to erratic error, > that the fix becomes 3 times more likely to lie outside than inside > the shape. > > How can either presumption be made? It seems to me that, a priori, > any round of sights may contain some extent of both types of error. > > Therefore the most useful approach is to eliminate erratic error at > source. This can be done via the comparison of a number of sights of > the same body taken over a few minutes with the slope caused by the > apparent rise/fall of the body observed over that period. > > Then, erratic error eliminated (to a usefully practical extent), any > remaining error, now assumed to be the constant type, can be dealt > with by bisecting those intersecting LOPs and finding the fix ... at > the centre of the shape. > > Et voila tout. That's it. > > > > > --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---