The Maximum Elevation Figure is the highest elevation in a quadrant of the sectional, including terrain and obstacles such as towers and trees, printed in feet above mean sea level. A large 4 and a small 1 means 4,100 feet MSL. The figure is not a flight altitude, and it is not an AGL height.
What the number is, and what a quadrant is
A quadrant on a sectional is the area bounded by the ticked lines that divide each 30 minutes of latitude and each 30 minutes of longitude. One MEF covers that box. It represents the highest terrain or obstacle in the quadrant, in feet MSL. It is not the lowest point in the box, and it is not an airport traffic pattern altitude.
The Chart Users' Guide rounds the result up to the next hundred-foot level and does not show the last two digits. A large 4 with a small 1 is 4,100 feet MSL. It is not 41 feet AGL, and it is not 410 feet AGL above the nearest airport. It is not a mandatory Part 107 ceiling. Section 107.51 still limits the aircraft to 400 feet AGL, or to 400 feet above a structure when you are within 400 feet of that structure. The MEF does not raise or replace that limit. How AGL and MSL differ is on the AGL versus MSL page.
The printed figure is conservative. It is based on the highest terrain or obstacle, plus the allowance the procedure below requires, then rounded up. It is not an operating authorization, and it is not a substitute for checking the actual terrain and the obstruction symbols in the quadrant. The FAA notes that the figures are not verified by field surveys. Confirm you are using a current chart so the MEF data is the latest issued.
Two procedures, not one "add 200 feet" rule
Aeronautical Information Specialists use two different methods. Which one applies depends on what the highest feature is.
When a man-made obstacle is more than 200 feet above the highest terrain in the quadrant, the steps are these. Determine the elevation of the top of the obstacle above mean sea level. Add the possible vertical error of the source material. That error is 100 feet, or one-half the contour interval when the interval on the source exceeds 200 feet. U.S. Geological Survey quadrangles with contour intervals as small as 10 feet are the maps normally used, so the added error in the published example is 100 feet. Round the result up to the next higher hundred-foot level. Do not add a separate 200-foot obstacle allowance in this case. The obstacle is already the feature being measured, and it already stands more than 200 feet above the surrounding terrain.
The Guide's man-made example runs like this. The obstacle top is 2,649 feet MSL. Possible obstacle error is 100 feet. The sum is 2,749. Raised to the following hundred-foot level, the MEF is 2,800 feet MSL. On the chart that would show as a large 2 and a small 8.
When a natural terrain feature, or a natural vertical obstacle such as a tree, is the highest feature in the quadrangle, the steps add one more number. Determine the elevation of the feature. Add the same possible vertical error of the source, 100 feet or half the contour interval when that interval exceeds 200 feet. Then add a 200-foot allowance for uncharted natural or man-made obstacles, because chart specifications do not require every obstacle below the minimum height to be portrayed. Then round up to the next higher hundred-foot level.
The Guide's natural-terrain example runs like this. The elevation is 13,161 feet MSL. Possible vertical error is 100 feet. The obstacle allowance is 200 feet. The sum is 13,461. Raised to the following hundred-foot level, the MEF is 13,500 feet MSL. The 200-foot allowance is part of this natural-feature method only. Collapsing both methods into "always add 200 feet, then round" misstates the man-made procedure.
Reading 4 and 1 on a test quadrant
A quadrant prints a large 4 and a small 1. Restore the two omitted zeros: 4,100 feet MSL. That is the quadrant's highest terrain or obstacle after the procedure above and the round-up. You do not fly at 4,100 feet because the number is large. You do not convert it to AGL. You use it as a warning that something in that 30-by-30-minute box reaches that high, and you then read the individual obstruction symbols, which separate MSL and AGL. Those symbols are on the obstruction heights page. The rest of the legend is on the sectional symbols page.
The trap
The wrong readings are a lowest-point elevation, a traffic-pattern altitude, an exact building height with no allowance, a Part 107 altitude authorization, or an AGL ceiling of 41 or 410 feet. The figure is a conservative MSL value for the highest feature in the quadrant, rounded up, with the last two digits omitted. A large 4 and a small 1 means 4,100 feet MSL, and it is not permission to fly there.
Practice quadrant readings on the free practice test. Chart reading is taught in the course.