A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Bob Goethe
Date: 2017 Feb 17, 12:43 -0800
You can download the 2002 edition of Bowditch from http://fer3.com/arc/navbooks2.html. Go to table 14 in the back, and you can get a dip correction for a "horizon" that is close to you because of land. If you can get a map of your area, and say..."When I look out at 270°, then land is 1.6 nautical miles from my position. So when I take a sight with my feet on the edge of the water, and my eye 5 feet above water level, I should use a dip correction of 2.4'.
I have taken a number of sights from the shore of a nearby lake. I go to Google Earth to sort out how far the shore is from me, at the compass bearing I am looking...and then I go to Bowditch to determine my dip.
I do this either by taking the sight first, going home, looking at Google Earth, going to Table 15 in Bowditch, and then reducing my sight...OR by planning my sight: determine where I will stand, and what my dip correction should be. In the latter case, I can do my sight reduction right on the shore.
You can see from the table that if your height-of-eye is 5 feet, and the land is 2.7 nm away from you, then you ARE looking at a true horizon. You may be seeing trees and hills on the land, but the actual point where the water meets the beach is over the horizon from you.
So on my lake, I know that from the west shore, any sights taken where the azimuth of the celestial object is between 74° and 109°, I am looking at a "true horizon" as the land I see is 3+ nautical miles from me. I need no further corrections of my dip than I would use if I were 100 miles offshore.