Rummaging through my closet, I came upon my old professor hat. Thought I’d put it on, and write a bit about sea level.
If you’re a hiker, you’re familiar with those USGS topo sheets showing, among other things, terrain relief. You probably also know those numbers you see on the elevation lines are all measurements of the vertical distance to sea level. Even if you’re not into hiking, you might know the elevation of some mountain, or the highest point in your state, or the levee down by the river; same thing for them, measured above sea level.
You probably also know that the sea level is rising. What does that do to all those numbers?
Well, nothing, actually.
It might seem that the level of the sea used to be constant, and any change is pretty recent, but there have always been fluctuations, both on a local and a global scale.
Think about something as seemingly simple as determining sea level at any given point on a shoreline. Do you measure it during high tide or low? Full moon or new? What about those little ripples of waves lapping the shore; is your measurement going to be taken at the maximum encroachment or the minimum?
Okay, fine, you say, measure all those points and use the average. That, in fact, was originally what was done, which is why the official elevation was always given as distance above mean sea level. Unfortunately, that doesn’t fix things. Because, you see, you have to ask yourself, mean sea level exactly where? The solution was to take means at various points, and average those, and so on.
What developed was a convention in which a statistical mean was taken as sea level, which didn’t correspond to actual sea level anywhere in particular. At this point, sea level was already an abstraction, but some respect was still given to the original concept, and means were kept as close as possible to actual sea levels. But it turned out that if you used a global mean, the numbers were too far off. In the end, we got elevations taken with respect to various regional reference points, or datums, around the world: the North American datum, the European datum, and so on.
Confused? How can sea level differ so much around the world?
First of all, the earth is not a sphere; it’s more like a lumpy egg. As a result, early suggestions to use the center of the earth as a reference were useless. Furthermore, water is not even level with respect to local elevations. Lake Huron, for example, is 5 centimeters higher at the south end than at the north end. This is partly due to the direction of water flow, but also in the difference in composition of what’s under it. The iron-rich substrates in the north are denser, and therefore exert a greater gravitational pull. These effects are compounded globally.
The result is the so-called equipotential system of sea level. Rather than using the physical measurement of the distance from a point above another point, mean sea level is now defined as an imaginary surface on which every point measures the same gravitational pull. The only concession to the actual level of the sea is in the name.
Of course, that used to be a very hard thing to measure. Thank goodness for GPS satellites.
(to be continued)