AGL vs MSL: How to Know the Proper Altitude Term

In this article, you will find information about AGL vs MSL altitudes and how to pilot drones correctly
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Last updatedLast updated: August 06, 2021

There are various factors you must consider when flying a drone. One of those factors is its altitude. In case you don’t know, drone altitude limitations may exist in your area based on federal restrictions. So, you’ll be limited to a certain altitude. Various units are used to report and interpret altitude numbers. Prominent among those units are AGL and MSL. As you can tell from their initials, they’re different. Hence, we’ll pit AGL vs MSL in a bid to explain the differences between them and how they affect their usage.

Sometimes, you’ll need to use AGL, other times, MSL will be the better choice, so you must understand how both units work. In this article, we’ll dig deep into the workings of AGL and MSL, thus helping you to understand how to interpret your drone altitude perfectly. 

AGL

You’ve probably been itching to know what AGL stands for. The acronym stands for Above Ground Level. Simply put, that’s the altitude measured from the ground below. No doubt, AGL is a commonly used unit in aviation and that tells that it’s quite important. Your drone’s altimeter uses AGL values when flying near its takeoff point. 

Thanks to the AGL altitude, a drone pilot can tell how far away the ground is but that doesn’t always work. If the drone flew too far away from the takeoff point, you could have trouble determining the AGL altitude. Pilots of manned aircraft typically coordinate with air traffic facilities to determine the altitude but a drone pilot doesn’t have that luxury. 

One significant disadvantage of the AGL altitude is its sole reliance on the altitude of a specific reference point which may deceive drone pilots that their machine is flying clear of obstacles. 

MSL

MSL means Mean Sea Level or more correctly, Above Mean Sea Level. As the name suggests, MSL altitude values are measured against the sea level. Hence, it remains a constant value without any regard for the terrain you are flying over. 

Of course, the sea level throughout the world is not exactly constant. However, the sea level’s universally accepted value was derived by studying the short-term and seasonal variations of tides over 19 years. 

Also, the atmospheric pressure is measured above standard sea level to calibrate altitude. Hence, MSL provides the effects of atmospheric conditions on your drone’s flight performance. 

Drone flight AGL vs MSL

AGL vs MSL: How to Know the Proper Altitude TermMSL vs AGL or ground level vs sea level, which takes the crown? We must admit that understanding the difference between MSL and AGL can be confusing but we’re about to simplify it for you. One significant difference between AGL and MSL is the level from which they’re measured. While AGL is the distance that a drone is above the ground below it, MSL is the distance that a drone is above seal level. 

For example, if you’re on the roof of a 50-foot tall building, you’re at 50 feet AGL. However, if a building is at 100 feet MSL, it doesn’t necessarily mean that it’s 100 feet tall. Can you tell the difference here? While a building at 50 feet AGL means the building is 50 feet tall, the logic doesn’t apply for MSL. Rather, MSL deals with the sea level.

Don’t be surprised to learn that a 100-foot tall building is at 150 feet MSL. That only means that the land it is built on is located 50 feet above mean sea level. With this understanding, you can easily convert MSL to AGL.

Moreover, MSL is considered to be a more reliable unit when flying a drone over areas with carried terrain because AGL can quickly change if you’re flying over mountains and hills with varying heights. Also, AGL only works accurately when the drone’s altitude is relatively low. However, MSL works well even in very high altitudes. So, drone pilots typically switch to MSL in such cases. 

Part 107 license

AGL vs MSL: How to Know the Proper Altitude TermAccording to the United States Federal Aviation Administration (FAA), you must obtain the administration’s Remote Pilot Certificate in order to fly your drone under the FAA's Small UAS Rule (Part 107). Trusted Source Become a Drone Pilot www.faa.gov

Before you can receive the license/certificate, you’ll need to complete the Part 107 Small UAS Initial (ALC-451) online training course. The course requires you to understand the operating requirements, regulations, and procedures (including AGL and MSL) for safely flying drones.

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Questions to consider

Let’s treat a couple of questions that simplify the difference between AGL and MSL below: 

Question 1

You get on an elevator from the ground floor of your 100-foot tall office building and it takes you right to the roof. What can you say about this? Obviously, you’re at 100 feet AGL because you’re 100 feet above the ground below you.

Question 2

You’re standing on the roof of a 150-foot apartment building that is at 180 feet MSL. What can you say about it at the first attempt? Since the ground in which the building is located is at 180 feet MSL and the building itself is 150-foot tall, you’re at 330 feet MSL. 

Final Thoughts

As a commercial drone pilot, you must know the difference between altitude values told in AGL and MSL because that helps you safely fly your drones. It also keeps you from going past altitude limits put in place by authorities. For example, the FAA’s maximum allowable altitude Trusted Source Fact Sheet – Small Unmanned Aircraft Systems (UAS) Regulations (Part 107) www.faa.gov is 400 feet above the ground. This is also the case in the United Kingdom per the country’s aviation regulator Trusted Source Please find below details of the CAA Publication you have selected. Dependent on availability, you are able to download the file, or purchase a printed copy. www.caa.co.uk

Besides, in comparing AGL vs MSL, you can tell that the former’s value is limited based on altitude but MSL offers a more accurate value regardless of altitude or terrain. Notwithstanding, both units are important and you should understand them if you’re piloting the best drones for roof inspections or even the best drones for fishing.

References

1.
Become a Drone Pilot
2.
Fact Sheet – Small Unmanned Aircraft Systems (UAS) Regulations (Part 107)
3.
Please find below details of the CAA Publication you have selected. Dependent on availability, you are able to download the file, or purchase a printed copy.
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