Next message: Frank Reed: "Re: dip, dip short, distance off with buildings, etc."
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
