What are MSL and AGL?
One aspect of drone operation that can be confusing is understanding the difference between MSL ans AGL. AGL and MSL are important measures the FAA uses to help pilots determine the altitude limits at which their drones can fly.
AGL, or above ground level, is the distance that an object is above the ground below it. For example, if you are standing on the roof of a building that is 100 feet tall, you are at 100 feet AGL.
MSL, or mean sea level, refers to the distance an object is above sea level. For example, if a building is at 200 feet MSL, that means that the building is 200 feet above mean sea level. This does not necessarily mean that the building is 200 feet tall, however—a building could be 50 feet tall (or 50 feet AGL), and at the same time 200 feet MSL because the land it is on is located 150 feet above mean sea level.
To commercially fly a drone, it is necessary to obtain a remote pilot certificate from the FAA. To receive this certificate, you must first pass the FAA’s remote pilot exam, which is commonly referred to as the Part 107 Test. On this test, you will encounter questions that will require you to have a good understanding of the differences between MSL and AGL.
Practice Questions about MSL and AGL
Below, we’ve provided you with a couple of sample questions (along with explanations about why each answer option is correct or incorrect) so that you can test yourself to see if you truly understand the difference between MSL vs. AGL.
Question 1: You climb up 13 flights of stairs to the roof of your apartment building, which is approximately 130 ft. tall. What can you know for sure?
A: You are at 130 ft. AGL
B: You are at 130 ft. MSL
C: You are at both 130 ft. AGL and MSL
D: You are at neither 130 ft. AGL or MSL
If you guessed A, you’re right! You know for certain that you are 130 ft. AGL because you are 130 ft. above the ground below you. Answer B and C are incorrect because you do not know for certain if you are 130 ft. MSL because you did not receive information about how high above mean sea level the land that the apartment building is located on is. D is also incorrect because you can know for sure that you at least one part is true—that you are 130 ft AGL.
Question 2: You are standing at the top of a 60 ft. tall rock that is 100 ft. MSL. Which of the following is true?
A: You are at 60 ft. MSL
B: You are at 100 ft. AGL
C: You are at 160 ft. AGL
D: You are at 160 ft. MSL
If you guessed A, you are wrong. You are not 60 ft. MSL because the information provided clearly states that the rock itself is 100 ft. MSL, and you are standing on top of the rock, which would make it impossible for you to be at only 60 ft. MSL. If you guessed B, you are also incorrect because the rock is only 60 ft. tall (60 ft. from the ground), which means it wouldn’t be possible for you to be at 100 ft. AGL. If you guessed C, you are still incorrect for the same reason as B. Which means, if you chose D, you are correct! You would be correct because the ground on which the rock is located is 100 ft. MSL… and if you add in the 60 ft. tall that the rock is, that would put you at 160 ft. MSL.
Now let’s try a question that you might see on the FAA exam.
Question 3: You are flying your drone, with FAA approval, within 1 mile of a local airport. The controlled airspace for this airport begins at 700 feet AGL. The elevation of the airfield is 145 feet MSL. At what altitude would your drone enter controlled airspace?
A: 700 ft. MSL
B: 150 ft. AGL
C: 850 ft. MSL
D: None of the above.
If you guessed A, you would be incorrect because the airport is already at 145 ft. MSL, so if you were flying at 700 ft. MSL, you would still be about 150 ft. below the controlled airspace level (700 ft. AGL). If you guessed B, you would also be incorrect because at 150 ft. AGL, you would be flying well below (550 ft.) the controlled airspace. If you guessed C, you are correct! This is because the airport is located on land that is at an elevation of 145 ft. MSL; if you add another 700 ft. to that (the distance between the ground and the start of the controlled airspace), you would be at about 850 ft. MSL. If you guessed D, you would be incorrect because we already know that C is the correct choice.
Looking for More?
Wondering what other types of questions you’ll run into on the Part 107 Test? Check out our blog for examples of questions, as well as detailed explanations that will give you an idea of what you’ll need to know for the exam.
If you’ve decided that you are interested in getting your remote pilot certificate and need help studying for the Part 107 exam, you might want to check out Drone Launch Academy’s comprehensive Part 107 prep course. The course has over 120 minutes of HD video instruction, practice quizzes, a downloadable e-book, and a practice final exam that simulates the real FAA test.
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If you have questions that you’d like to speak with an expert about, please feel free to reach out to us.