NavList:

Yes, that is a good question. :)

Your height of eye should be measured be measured relative to the actual water level. So if it's low tide in a region with an extreme tidal range like coastal New Hampshire, then you should use a higher height when the tide is out. To complicate matters, the horizon is composed of overlapping wave crests at the horizon, so what you really want is your height above the wave crests. Since wave heights can be rather 'chaotic', this becomes a game of estimation. If there's 9 feet of low tide below the bottom of the pier you're standing on with 3 foot swells rolling in, then you want to count your height of eye from 6 feet below the base of the pier. This once again illustrates that the horizon is the greatest source of uncertainty in celestial navigation (for a typical observer with a good, properly-adjusted sextant).

Frank Reed