What Is True Altitude And How Do You Measure It?

Knowing your altitude while flying is incredibly important. But there are different types to know, so learn all about true altitude in this complete guide.

If you talk to anyone who’s experienced in flying, they’ll all tell you that knowing how high you’re flying is important at all times. But if you’ve looked into it, you may know that there isn’t just one type of altitude for you to keep in mind. So what exactly is true altitude and how can you measure it?

Simply stated, true altitude is how high you are above mean sea level. Since the mean sea level is independent of the terrain you’re flying over, true altitude is not indicative of how high you actually are above the ground. It can be measured with radar or calculated from indicated altitude.

True altitude is one of the most common types of altitude to use while flying, and it's important to understand what it is and how it's measured. In this article, we will discuss what true altitude is, why it's important, and how it's measured. We'll also compare true altitude to other types of altitudes such as indicated altitude and absolute altitude. So, let's get started!

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‍What is True Altitude?

To someone that's just getting into aviation or to anyone that isn't into aviation at all, it might seem like there is just one altitude. I know that before I got into aviation, I thought altitude and height were the same things, and there was only one — just called altitude — that meant how high the plane was flying. But as I got more into it (just like you are now!), I quickly learned that there are many different altitudes that mean all sorts of different things.

So what is true altitude?

Unlike some things in aviation, there is no subjectivity to this definition. It is spelled out for us by the Federal Aviation Administration (FAA) as being the height above mean sea level (MSL). This of course indicates that you have to know what mean sea level is, which is actually what is generally just called "sea level". It's the average surface level of the Earth's major coastal bodies of water and is used around the world as a standard measurement datum.

This might seem like a strange thing to use as a reference point for flying since you might not be flying anywhere near the water, so what does sea level have to do with anything? Well, that's exactly why it's important to know the different types of altitudes. True altitude in particular is just the height above MSL, but other altitudes may not use it at all.

How is True Altitude Measured?

Now that you know what true altitude is, let's talk about how you can figure out what it is. For all intents and purposes, there are two main ways to determine your true altitude: direct measurement and calculating it using other known variables.

Measuring True Altitude Using Radar Over Water

The easiest and most direct way to measure your true altitude is with the use of radar. This only works, however, if you're flying over a large body of water like an ocean. The reason for this is that the radar will directly measure your height above the water, but recall that it has to be MSL, not just the surface of any old lake or river.

If your aircraft has the equipment necessary for this type of measurement and you're flying over an applicable body of water, it couldn't be any easier. The equipment will send out the radar signal, wait for it to bounce back, and automatically calculate your true altitude for you. Voila, that's it!

Calculating True Altitude From Indicated Altitude

Since true altitude is defined as your height above sea level, it might feel impossible to know without flying over the water as in the method above. But thankfully, there's another way to calculate it that you can do no matter where you're flying. And that's by using your indicated altitude and applying the following formula:

TA = IA + (ISA Deviation*IA*4/1000)

In the above equation, TA = True Altitude, IA = Indicated Altitude, and ISA Deviation is the deviation in atmospheric conditions (temperature in degrees Celsius). Technically, the formula should be 1/273 instead of 1/250 (4/1000), but the latter is often used since it's easier to calculate and it's the basis of the meteorological 4% rule.

In other words, this formula basically means that from indicated altitude to true altitude, there is roughly a loss of 4 feet for every deviation of 1°C for each 1,000 feet of altitude. So if you're indicated altitude is 30,000 feet and your SAT at that altitude is -39°C. ISA at FL300 (30,000 feet) is about -44°C, so this gives us an ISA Deviation of -5°C. Let's plug those numbers into our formula:

TA = 30,000 + (-5 * 30,000 * 4/1000) = 30,000 + (-600) = 29,400 feet

It might seem like a complicated formula at first glance, but as you can see, it's actually super easy to apply once you have all the right variables! But that's just it, this formula rests on the fact that you know your indicated altitude, but even is that?

What is Indicated Altitude?

Thankfully, indicated altitude is arguably the easiest of the bunch to understand. It's simply what your altimeter reads when it's set to the proper atmospheric conditions (typically standard conditions). This is pretty much just the altitude that you're used to seeing on your typical aviation altimeter, so nothing too out of the ordinary here.

In other words, this is the actual altitude that you can read from the equipment inside the cabin of your aircraft. This is still not the one that's going to be changing constantly based on where you're flying and the terrain below you, but we'll get to that one at the end of this piece!

Using an Altimeter to Measure Indicated Altitude

As we just talked about, knowing your indicated altitude is super easy because it's just what your altimeter reads. And measuring that is as simple as using an altimeter!

An altimeter is a mechanical or electronic instrument used to measure the altitude of an aircraft or other object above a fixed level. The most common type in use today is the barometric altimeter, which measures pressure to determine altitude. Oftentimes, the altimeter will be set to standard atmospheric conditions so that it will be applicable in most cases.

Is Absolute Altitude the Same as True Altitude?

Lastly, let's touch on absolute altitude since people often get it confused with true altitude. After all, the words "absolute" and "true" seem to mean similar things in most cases, so do these altitudes actually refer to the same thing? Unfortunately, no. Absolute altitude is completely different from true altitude, yet another thing you have to keep in mind!

Absolute altitude is defined as the vertical distance (height) above ground level. In other words, it's your height above the ground below you at any given time, and will always be changing depending on the terrain you're flying over. For example, if you're flying through Colorado and passing over the mountains, your absolute altitude will change as the ground level below you does.

So if you're flying over ever-changing terrain, this can often be the most important one to keep in mind, but we'll dive into absolute altitude much more deeply in another article!