A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2018 Jun 12, 18:35 -0700
Jackson, you asked:
"What is the shortest practical dip short?"
A nautical mile is a good lower limit. The problem is that both the distance to the opposite shore as well as your height of eye need to be rather exact in order to use dip short. The best way to assess this is to experiment, either with calculated dip short or with observed dip short.
There are lots and lots of tools available that will calculate dip short for you. Since they haven't been mentioned recently, you should try out Peter Hakel's extensive collection of spreadsheets at navigation-spreadsheets.com. You can run them on practically any device though you will first need to install some spreadsheet app (the files themselves are in the most compatible format so you don't need to be picky about the software itself). Here's the direct link to his dip short calculator spreadsheet: http://www.navigation-spreadsheets.com/uploads/dip_short.xls. This is a very simple calculation, and you certainly don't need a spreadsheet for it, but it's worth exploring this collection of tools generally.
Once you have it installed, try varying the inputs. Try a height of eye of 25 feet and a distance to the obstruction (opposite shore) of 1.0 n.m. Then vary either of these parameters by some reasonable uncertainty like 10%. What happens to the dip short correction? Can you accept the resulting uncertainty within the context of practical navigation? Well, that all depends on what we mean by practical, doesn't it?! Heh. And remember, this is error that exists in addition to any other errors in your observation.
And how do you know the distance to the opposite shore to better than 10% accuracy anyway? Because you looked on a chart? Well then since you know where you are, you can just measure the dip short by taking a sight. Whatever is not accounted for, that's your observed dip short correction. No tables or calculations required. And here, too, you can experiment. Try sights from one foot lower in terms of height of eye. What observed dip short correction do you find then?
But I'll stick my advice: one nautical mile or greater, not less than that.
Clockwork Mapping / ReedNavigation.com
Conanicut Island USA