How High?

Portland Head Light, by Peter M. Mason
from Maine Lighthouses Illustrated Map & Guide

I didn't get clued into the different ways to measure a lighthouse till I decided that we should include focal heights along with tower heights in our Maine Lighthouses Illustrated Map & Guide. On some websites, I discovered, a light's distance above sea level (focal height) was shorter than the tower; on others it was higher.

What the...?

That's when I learned that some sources cite the tower height without the lantern atop, others with. (See How Tall?)

OK, I figured, I could sort of fudge the tower heights, but sea level is a constant, right?

WRONG. There's mean low tide, mean high tide, and somewhere in between ("just plain mean").

After racking my brains and drinking yet another tub o' latte, I made an executive/editorial decision: We'd use "sea level" and let other people figure out its exact meaning.