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One of the most important numbers on a sectional is the Maximum Elevation Figure, or MEF.
The MEF depicts the lowest altitude you can fly and still be assured to clear every obstacle within that quadrant.
MEF's are calculated two ways, one for natural obstacles, and one for man made obstacles. Each method ensures the highest level of safety and that as long as you stay above MEF you will be clear of all obstacles.
When a man made obstacle is the highest obstacle within a quadrant, the FAA uses the following formula to calculate the MEF:
Obstacle height (MSL) + 100ft
Round up to the next highest 100 feet and you have the MEF. For example:
Obstacle 750ft + 100ft = 850ft - Round up to 900ft
When a natural obstacle is the highest obstacle in a quadrant the FAA uses a different formula for extra safety:
Obstacle Height + 100ft + 200ft
Again, round the result up to the next 100ft and you have the MEF. For Example:
Mountain 3250ft + 100ft + 200ft = 3550ft - Round up to 3600ft
The 100 ft addition to both altitudes to ensure proper clearance when there has been an error in measurement, a slight deviation from plans, or for any other reason that the obstacle may be a little taller than originally measured.
The 200ft addition for natural obstacles provides extra clearance due to vegetation growth as trees get taller, buildings or towers that may have been constructed, or other items.
In areas where the contour interval for that specific quadrant is greater than 200ft, half of the contour interval is added instead of 100ft. So if that specific interval is 300ft, add 150ft instead of 100ft. These cases are very rare and occur only where there is a very steep elevation change.
The MEF is great tool to ensure you clear obstacles. If you are flying below MEF, make sure that you know where all the obstacles are. Towers, wires, and other man made structures are difficult to see from the air.
The MEF is also a VFR only number. It can be used in an emergency for IFR flight, but it is strictly designed for aircraft flying VFR.
Please let me know if you have any questions, and add your comments below!