Part 107 exam AGL, MSL, and surface level

[codydoerfler]

I have been taking practice exams and I always miss the questions that pertain to MSL, and AGL. I am confused as to when MSL is used and AGL is used.

Example question

(Refer to FAA-CT-8080-2G, Figure 21) Find the radio tower that is directly to the south of Minot International Airport (MOT) in Area 1. [Note: The tower is along interstate 83, with a mean sea level altitude of 2,179 feet.] At what altitude above this obstruction is the UAV operator allowed to fly (ignoring any potential airspace restrictions for now)?

750 feet mean sea level (MSL)
2,579 feet mean sea level (MSL)
798 feet above ground level (AGL)

The correct answer is 798 feet above ground level (AGL)

I just can't figure out when MSL and AGL are used.

Thanks in Advance.

 

[STGill]

This is one of those questions with a lot of distractors. MSL vs AGL can be confusing, just best to slow down and make sure which one is being asked about. In this case, the tower altitude is shown as 2179 with (398) just below. When you see that, it's showing you MSL with AGL in parentheses. With these types of questions you might want to work backwards. You know your altitude limit is 400 feet. Does adding or subtracting 400 from either 2179 or 398 get to an answer? In this case I think I was also confused because it looks like 2579 MSL should be correct, but I think it isn't because we don't refer to UAS limits in MSL, but in AGL due to terrain and building height variability.

There were a few questions on the prep tests I felt I just had to memorize but didn't entirely understand, I think the questions are sometimes a little unclear since they were adapted from manned aircraft testing.

 

[N42742]

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[codydoerfler]

The video helped, thank you!