A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2017 Apr 22, 11:52 -0700
Mike, you wrote:
"surely the visible horizon is made up of crests only. No need to consider this factor in my opinion."
Quite right. In conditions where there are "choppy" swells, the best you can do is estimate in any case, but the height that counts is the height above the mean surface of wavetops that merge to form the visible horizon. What source gave the advice that you mentioned? The main thing to worry about with swells is to make sure your height of eye is positive! You want to be above the average wavetops. This can even be a concern when you're doing practice sights from shore if you're right at sea level and there are big swells offshore.
By the way, I notice that you wrote dip as "DIP". Since it's not an abbreviation or acronym, that uppercasing is not necessary. :)