Originally Posted by steve_hendry
So, 'at the moment,' what was the state of the tide? It's actually the geoid that is close to mean sea level. The ellipsoid is a simpler shape, an approximation to the geoid, that happens to stay within +/- 100 meters of the geoid throughout the world. The satellites don't have the geoid info, it's built into your GPS, i.e., if it knows the position (lat, long, ht) relative to the ellipsoid, it has an algorithm to compute position on the geoid.
The datum the tide is measured from ("0" tide) varies from region to region. It is typically the same as the sounding datum on the charts
for your area. That means more often than not it isn't Mean Sea Level. As hummingway notes, in his region it is LAT. On the US West Coast
(including Alaska) it is Mean Lower Low Water
, the average of the lowest of the two tides each day. That's why the tide tables show fewer minus tides than positive tides.
The Geoid is a gravitational equipotential surface. That means that "vertical" is always at right angles to the Geoid. Latitude and Longitude from astronomical fixes are measured relative to vertical. But the geoid is kind of "lumpy", since variations in density of the earth's crust effect gravity. Lumpy geoids are hard to deal with mathematically. Various spheroids have been designed to approximate the geoid and make it easier to deal with mathematically.
It can all get rather confusing. See Nigel Calder's book on chart reading for the best simple explanation I've seen of the Geoid, Spheroids, and chart datums.
"Gravity - It's the Law!"