Quote Originally Posted by brdad View Post
So, the elevation of your waypoint is still, -15 feet, reguardless of the ocean level, correct?


On the Maine Harbors web site, they say "The tide data on our charts is relative to mean lower low water (MLLW - 0.0') which is the average of the lower (of the two) low water heights of each tidal day. The unit of measurement is feet (and 10ths of a foot)."

But either way, it still seems to me the water depth on a map is from mean sea level, so that would determine the elevation.

See what you started by asking for highest/lowest? ;-)
Yes, it does get complicated.

How is the elevation of the map defined? If it was defined as the elevation above MLLW, then the elevation of the sand on that day was -1.9. This fact requires the map datum to be 13.1 feet below MLLW, which seems like an odd choice.

More likely, the water depth on the map is wrong. It may have been correct on the day the map data was collected! Don't forget, the elevation of the sand is constantly changing, so there is no way to keep the map up to date.