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: Frank Reed CT
Date: 2006 Jan 13, 01:41 EST
From: Frank Reed CT
Date: 2006 Jan 13, 01:41 EST
Michael B, you wrote: "My understanding is that terrestrial surveyors of old ( and RN surveyors of land features)got their local horizontal datum from a highly portable reference source, i.e. a leather bag of mercury. Exactly as the technique for finding the position of the back garden using a paint roller tray full of oil. In that type of situation, what's dip and near horizon refraction got to do with anything...why not do as the oldies did and avoid the problem...?" If you're on land, sure, you can use an artificial horizon to get the local horizontal datum, just as you say. But that's only part of the issue. First, that clearly doesn't do you any good if you're on a boat. But second, even after you've established the horizontal, all observations of distant objects are significantly affected by terrestrial refraction, and it is a variable quantity. So even if you have some alternate means of getting the horizontal, you still have to deal with the issues of lapse rates and temperature inversions. All of these things --dip, dip short, range of visibility, distance by measuring heights of objects beyond the horizon, etc.-- are tabulated in the navigation manuals, like Bowditch. And navigators have used these tables with little reason to doubt them in most cases. Not that they haven't been warned. In fact, there is almost always some advice in the navigation manuals that the navigator should watch out for variable refraction. Unfortunately, this advice has been qualitative and sometimes contradictory. Although "traditional navigation" has little remaining practical importance, we do at least now have the luxury of going back and cleaning up some of the material that has been done a bit sloppily in the past. -FER 42.0N 87.7W, or 41.4N 72.1W. www.HistoricalAtlas.com/lunars