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**Worked example — Class C ceiling math.** - Sectional fraction **41 / 16** on a Class C ring → ceiling **4 100 MSL**, floor **1 600 MSL**. - Airport field elevation labeled **318' MSL** on the chart. - Callout **718' MSL** = 318 (ground) + **400' AGL** limit = max legal drone altitude MSL at that spot. - **Takeaway:** Part 107's 400 AGL cap becomes a site-specific MSL number once you know the terrain elevation. Plan in AGL, report in MSL.

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