Questions from the Cockpit: What’s so mean about mean sea level?

Cassandra, working on her Sport Pilot ticket on the Florida coast, writes: I’ve just started my flight training, and have learned about the two different ways that altitude is expressed: AGL and MSL. Above Ground Level is self-explanatory, but I’m having a harder time wrapping my head around Mean Sea Level. I mean, I get that it’s the altitude above sea level, but what’s the “mean” all about?

Back in the day, flight instructors liked to mess with their student’s heads with all the different measurements, so students started calling the less logical of the two altitude measurements “mean” because their instructors were being mean, and the label simply stuck.

Sorry, I couldn’t resist. But your question is a good one and a lot more complex than you might expect.

Here’s the deal: For generations, mapmakers have needed some sort of baseline from which to measure the height of terrain. As the land starts at the ocean, the level of the sea — at first blush — seems like a logical floor from which to measure.

But there are problems. The first is obvious to anyone who’s lived on the coast, or even simply lain in the sand soaking up sun for any period of hours, and that’s the tide. The ocean rises and falls. But that’s not the half of it. Atmospheric pressure can change the level of the sea, too, as can wind and other factors.

So what is required, as the sea is dynamic, is an average level of the sea. In this instance, the word “mean” is used in its mathematical sense, which you may recall from high school math class as simply a technical term for “average.” So mean sea level is another way of saying average sea level.

OK, so that makes sense. But it begs the question: How do you calculate the average level of the sea, given all those variables I just mentioned?

Well, back in the day, they basically stuck a yardstick in the ocean and measured it. Every hour. Every day. For years. For 19 years, to be exact. Apparently that’s the length of time required to compensate for variations in the moon’s orbit. Then you simply add up the 166,440 measurements you took, and divide by that same number to get the mean — or average — sea level, and you’re good to go.

Thinking of it in pilot terms, Mean Sea Level is the reference datum for aeronautical charts — the spot from which you measure the elevation of the terrain, as well as the reference datum for altitude reporting.

As a student pilot, you’ll soon learn about reference datums for weight and balance calculations. That’s the location you use as the “zero point” when measuring the location of a load inside an airplane.

As pilots who fly a lot of airplanes quickly learn, the location of these datums varies from model to model. Sometimes it’s the firewall. Sometimes the spinner. Sometimes the wing root. Sometimes off in space in front of the airplane, because it doesn’t matter, it’s just a reference, an agreed upon starting place.

The early mapmakers had a similar problem: No one agreed on where to measure sea level from. In England, the measurements were originally taken at the Victoria Dock in Liverpool. But, of course, the French used another spot. As did the Spanish. As did the Russians.

As we get into aviation this matters because, as it turns out, across the coastlines of the planet sea levels are — believe it or not — altered by the density of the rock beneath. This means you don’t get the same average sea level when you calculate it in different locations.

And while we can live with different datums with different airplanes, when it comes to worldwide maps, I think you can see that we really need to get everyone on the same page. Are we going to measure from the firewall or the spinner? Victoria Dock or the Kronstadt Sea-Gauge?

This need for a global standard became even more pressing in more modern times, especially once we got into GPS systems. We can’t have everyone measuring from a different spot.

I won’t bore you with all of the history, but while mathematicians and mapmakers had been grappling with the lack of a universal mean for the planetary sea level since the 19th Century, it wasn’t until the dawn of the Space Age that any serious international progress was made.

Initially, a global standard was created by simply averaging various traditional measurements taken in different locations, but as science grew, so did our understanding of our not-so-round-after-all planet. By the mid-1960s mathematical models of the planet started being used to calculate a reference datum sea level. Like the real planet, these models are flattened spheres called ellipsoids. You can think of these ellipsoid models as prettied-up mathematical models of the earth. A perfect earth, as it were. Nice and smooth. All averaged out. Really, we should call it NSL, for Nice Sea Level, rather than Mean Sea Level.

We’ve been through a fair number of these ellipsoid models, the current one called WGS 84, for World Geodetic System, with the 84 a reference to the year 1984 when it was first adopted. Of course, it’s been through a number of updates since then, most recently in January 2021.

GPS systems use an ellipsoid coordinate system created by this model for both their horizontal and vertical datums. In other words, like sea level itself, the “exact” location on the planet displayed on your GPS is more fiction than fact. It’s measured off of a perfect model of an imperfect planet, not off the overly complex planet itself.

Now, you might think that creating and maintaining this key global resource for navigation would be a United Nations thing, but you’d be wrong. It’s maintained by the United States National Geospatial-Intelligence Agency or NGA. Never heard of it? Hey, it won an award from the Partnership for Public Service as one of the Best Places to Work in the federal government.

Don’t feel bad, I had never heard of it either. But every time you unfold a map, or turn on your GPS, you owe your data to the 14,500 men and women of the NGA.

And based on the best place to work award, I’m guessing that they’re no meanies.