What is a common altitude reference for obstacles on charts?

The correct answer is MSL, which stands for Mean Sea Level. This altitude reference is commonly used on aeronautical charts to provide a standard measure for obstacles and terrain. Using MSL allows pilots to determine the elevation of structures, terrain, and other obstacles relative to a common baseline. This is crucial for safe navigation and obstacle avoidance during flight operations.

AGL, or Above Ground Level, measures altitude relative to the underlying terrain and can vary significantly depending on the landscape. Obstacles might be listed with AGL, but MSL is preferred for standardized charting. HTL, which typically refers to Height Above Threshold Level, is not used as a common altitude reference for obstacles on charts. Thus, MSL is the most reliable and consistent reference for understanding altitude as it relates to obstacles on aeronautical charts.