4 (Refer to FAA-CT-8080-2G, Figure 23, area 3.) What is the floor of the Savannah Class C airspace at the shelf area (outer circle)?
B. 1,300 feet MSL.
Chart Tip:
Class B airspace is depicted with blue rings, and Class C airspace is depicted in magenta.
When elevations are given two numbers, like 341 over (311), the top number is MSL and the bottom number in parenthesis is AGL.
In this chart section, the Class C airspace is typical. The inner ring is five nautical miles in diameter, and the outer ring is ten nautical miles. (Measured from the geographic center of the airport.) The altitudes are given in magenta. The inner ring extends from the surface to 4,100 ft. MSL, and the outer ring begins at 1,300 ft MSL and also extends to 4,100 ft. MSL.
Class C Airspace is shown in abbreviated form on WACs. Sectionals and TACs show Class C in greater detail. The MSL ceiling and floor altitudes of each sector are shown in solid magenta figures with the last two zeros eliminated.  (FAA Aeronautical Chart User’s Guide, Page 9)
This is correct.
Why is this important to the sUAS pilot?
A sUAS pilot needs to know where restricted airspace begins. Since the rules do allow you to fly up to 400 ft above a structure, a really tall structure, like a Television tower, could put your flight into controlled or restricted airspace. The tower circled in red at the bottom of the chart section is a perfect example of this. The top of the tower is at 6,182ft. MSL (320 ft AGL). The Class-C airspace begins at 7,500 ft. MSL.
Subtract the tower elevation in AGL from the MSL to get the MSL elevation at the ground.  In this example 5,862 ft MSL is zero ft AGL.  Your UAS at 400 ft above the tower is at 720 ft AGL.  Add this to the ground elevation in MSL (5,862 ft)  and you get 6,582 ft MSL.  So, when your UAS is reporting an altitude of 720 ft AGL, it is at 6,582 ft MSL, which is below the floor of the Class C airspace at 7,500 ft MSL.
It is perfectly legal to fly a drone on a tower inspection mission up to 400 ft over while within 400 ft of an obstacle (§ 107.51(b)(1) and (2)), but at this location your aircraft would be inside Class C airspace.
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