What Is MSL in Aviation?

However, it is not a measurement that describes the space above the ground and below the pilot. That data point is called AGL, which is short for above ground level. 

AGL references how far the surface below a pilot is, which is often thousands of feet above the mean sea level or MSL. This article will take a look at how to calculate MSL and what it’s used for in terms of navigational safety alongside AGL, how MSL in aviation works, and other factors. 

Sea levels change regularly with tides, temperatures, rainfall, and atmospheric conditions, and are continuing to rise year over year. With that, an exact sea level is almost impossible to depend on, so the aviation community (as well as many others) agrees upon an MSL. 

MSL is a global measurement, meaning it doesn’t change from ocean to ocean, or airspace to airspace. Teams use various measurements from the previous 19 years, taking into account tides, seasonal variations, the different levels of each ocean (they do vary slightly), and yearly risings to determine the MSL. Generally, it changes by less than a tenth of an inch from year to year, but for pilots, precision is everything. 

Knowing exactly what MSL is in aviation makes it easier to understand the answers to questions like these! 

Sometimes referred to as the true altitude, MSL is a fairly constant measurement that pilots can read to ensure they are at the correct altitude during navigation. The precision of altimeters isn’t always perfect. 

That’s because pilots fly over varying landscapes during a flight. So, safe navigation and flight planning is the easy answer to that problem.

MSL measures an aircraft’s true altitude, but MSL is not synonymous with true altitude. The true altitude of an aircraft is its height above the MSL.

An altimeter measures the height an aircraft is above the ground below, and registers it as a value above mean sea level. If an altimter is set to 0 feet while on the ground about to depart, however, it could then be used to read directly the airplane’s height above ground level, or AGL.

Sectionals display MSL measurements the majority of the time, but heights in AGL appear in parentheses accompanying obstacles such as towers, to provide a more instantaneous value for the pilot to reference while flying over the charted area.

Sea levels change every year, and though it is by very small increments, if you’re not correctly calibrated to the current MSL, your altitude measurements could be off. 

Both of these measurements are very important for safe navigation, but the easiest way to explain why they are helpful is that AGL helps pilots determine how close they are to the ground below, and MSL is the aircraft height relative to a predetermined zero point that is known as... The mean sea level!

Determining the earth’s mean sea level is no easy task, but luckily for pilots, they don’t have to fully understand how the number is formed in order to use it correctly. It aids in safe aerial navigation among other tasks. 

When used with other altitude readings, like AGL values from the chart, MSL is a must for maintaining altitude during a flight plan. For more on the how, what, where, when, and why of aviation, consider subscribing to the FLYING today!