What does the bold number in the center of a sectional chart grid square mean?

That number is the Maximum Elevation Figure (MEF). It represents the highest known terrain or man-made obstacle in that grid square, plus a safety buffer calculated by the FAA, expressed in feet above mean sea level. A large digit followed by a smaller digit reads in thousands and hundreds - for example, 4 and 2 equals 4,200 feet MSL.

How does the FAA calculate the MEF?

For terrain and obstacles with known heights, the FAA adds 200 feet above the highest feature in the grid square, then rounds up to the next 100-foot increment. For man-made obstacles with unknown heights, the FAA assumes a 200-foot AGL structure, adds that to the terrain elevation, and rounds up. The final rounded figure is the printed MEF.

Is the MEF the same as a spot elevation on the sectional?

No. A spot elevation marks the precise elevation of a specific geographic point. The MEF covers the entire grid square and already includes the FAA's safety buffer above the highest feature in that area. Use spot elevations for terrain awareness; use the MEF for altitude planning decisions.

Why does the MEF matter more at night?

At night, you lose the visual depth perception and terrain awareness cues that help during daylight. Lit obstacles can be difficult to distinguish against ground lights, and rising terrain may not be apparent until you are dangerously close. A thorough night MEF check should also cover a 10–15 mile buffer on either side of your planned route to account for the possibility of drifting off course.

The Maximum Elevation Figure, the Bold Number in Every Sectional Grid Square, and the Altitude Trap That Catches VFR Pilots Off Guard on Cross-Country Day

The Maximum Elevation Figure (MEF) is the bold number printed in the center of every grid square on a sectional aeronautical chart. It represents the highest known terrain or obstacle in that square, plus a safety buffer, and it is one of the most actionable pieces of altitude information available to a VFR pilot. Checking every MEF along your planned route before selecting a cruising altitude is a fundamental planning step that primary training often underemphasizes.

What Is the Maximum Elevation Figure on a Sectional Chart?

A sectional chart divides its coverage area into a grid of rectangular boxes formed by lines of latitude and longitude, each roughly 30 nautical miles on a side. Every grid square contains a bold number - usually in dark blue or black - at its center. That number is the MEF.

The FAA calculates the MEF by surveying all terrain and man-made obstacles within the square, identifying the highest one, adding a safety buffer, and rounding up. The result is expressed in thousands and hundreds of feet above mean sea level (MSL).

When reading the MEF, the large digit represents thousands of feet and the smaller digit represents hundreds. A large 4 followed by a smaller 2 reads as 4,200 feet MSL.

How Is the MEF Calculated?

The FAA uses a specific methodology depending on the type of obstacle:

Terrain: Take the highest terrain elevation in the square, add 200 feet, then round up to the next 100-foot increment.
Man-made obstacles with known heights (towers, antennae, buildings): Apply the same 200-foot buffer and round up.
Man-made obstacles with unknown heights: The FAA assumes a structure height of 200 feet AGL, adds that to the underlying terrain elevation, then rounds up.

The MEF is not an arbitrary conservative guess - it is the product of a defined calculation. That calculation has a margin built in, but it is a finite margin, and it assumes all obstacles in the square have been charted.

What Does the MEF Actually Tell You - and What Doesn’t It?

The MEF tells you, specifically and precisely, the minimum altitude at which you can fly through that grid square and remain above all known terrain and obstacles. If your cruising altitude is at or above the MEF, you clear everything the FAA has charted in that piece of sky.

What the MEF does not do:

It does not guarantee cloud clearance or VFR weather conditions.
It is not a minimum safe altitude in the IFR sense - it does not add obstruction clearance for instrument approaches.
It does not account for obstacles that appeared after the chart’s publication date.

For VFR cross-country planning, it remains one of the most directly useful numbers on the chart.

How Does the MEF Differ from Spot Elevations?

Sectional charts also display spot elevations - smaller numbers scattered near mountain peaks and prominent terrain features. A spot elevation gives the precise elevation of that specific geographic point.

The MEF is different in two important ways. First, it covers the entire grid square, not a single point. Second, it already includes the safety buffer above the highest feature. When making altitude decisions for a cross-country flight, use the MEF - not spot elevations - as your reference.

How Do I Use the MEF When Planning a Cross-Country Flight?

Build the MEF check into your planning sequence as a deliberate step, not an afterthought. Here is where it fits:

Draw your course line on the sectional.
Identify every grid square the route crosses.
Note the MEF for each square - write them down in your flight log or scratch notes.
Find the highest MEF along the entire route. That is your MEF floor.
Select a cruising altitude that clears that floor by a comfortable margin.
Confirm your chosen altitude also satisfies FAR 91.159 (the hemispherical altitude rules).

This process takes approximately 90 seconds. It does not require deep analysis - just a deliberate scan before you leave the ground.

How Much Clearance Above the MEF Is Enough?

There is no regulatory minimum clearance above an MEF for VFR flight, but a practical working margin is 500 to 1,000 feet above the highest MEF along your route. That margin provides buffer for:

Measurement uncertainties in the charted data
Structures that are technically charted but easy to miss visually
The divided attention that comes with cross-country flying - navigation, weather monitoring, fuel management, and radio calls all competing simultaneously

Legal clearance and practical safety margin are not the same thing.

How Does MEF Planning Interact with the Hemispherical Altitude Rules?

FAR 91.159 requires VFR aircraft flying above 3,000 feet AGL to use hemispherical altitudes:

Magnetic courses 0° – 179° (eastbound): Fly at odd thousands plus 500 feet (3,500 / 5,500 / 7,500 / etc.)
Magnetic courses 180° – 359° (westbound): Fly at even thousands plus 500 feet (4,500 / 6,500 / 8,500 / etc.)

These two requirements must be satisfied simultaneously. A pilot planning a westbound cross-country with an MEF floor of 3,600 feet cannot simply choose 4,000 feet - that altitude has no 500-foot buffer and does not comply with 91.159. The next legal westbound altitude is 4,500 feet. Practice running this exercise before your checkride so the logic is automatic.

What Should I Know About MEF Checks for Night Cross-Country Flying?

The Private Pilot ACS includes night flight requirements, and night cross-country planning warrants a more thorough MEF check than a daytime flight over familiar terrain.

At night, lit towers and antennae become harder to identify against a background of ground lights. Depth perception cues that work in daylight are largely absent. Rising terrain below you does not present itself visually the way it does during the day.

Two specific practices for night MEF planning:

Be deliberate and thorough - not a quick once-over.
Check MEFs in a 10- to 15-mile buffer zone on either side of your planned route. If you drift off course at night, which is easier to do when ground references are sparse, your altitude should still provide terrain clearance in the adjacent grid squares.

How Does the MEF Relate to IFR Obstacle Clearance Concepts?

Instrument pilots use a related concept on IFR enroute charts called the OROCA (Off-Route Obstacle Clearance Altitude). The OROCA uses the same underlying logic as the MEF - highest obstacle plus a buffer - but with larger margins designed for flight without visual reference:

1,000 feet above the highest obstacle in non-mountainous terrain
2,000 feet above the highest obstacle in designated mountainous terrain

The MEF is the VFR pilot’s equivalent. The OROCA simply reflects the larger margins required when you cannot see what you are clearing.

What Does an Examiner Expect During the Oral Exam?

The Private Pilot ACS cross-country planning task requires demonstrating that you can use aeronautical charts to determine appropriate terrain and obstacle clearance. A prepared answer to “how did you determine your cruising altitude?” covers three elements:

You checked the MEF for each grid square along your planned route.
You identified the highest MEF on the route and selected an altitude that provides comfortable clearance above it - stating the specific value and location.
Your chosen altitude satisfies both the MEF floor and the hemispherical altitude rules under FAR 91.159.

That answer demonstrates you understand why the altitude was chosen, not just that you picked a number that sounded reasonable.

Why Does Chart Currency Matter for MEF Accuracy?

Sectional aeronautical charts are updated on a 56-day cycle. Terrain is stable, but man-made obstacles - towers, wind turbines, communication antennae - are constructed continuously. An out-of-date chart may not reflect a structure built since the last publication cycle.

FAR 91.5 requires current charts as a matter of regulation, but the safety case for currency is direct: an uncurrent chart means the MEF may understate actual obstacle heights in a given square. If you use an Electronic Flight Bag (EFB), verify that your chart subscription is active and that you are not flying with a cached chart more than 56 days old.

Key Takeaways

The MEF is the highest terrain or obstacle in a sectional grid square plus a calculated safety buffer, expressed in feet MSL. It is printed bold in the center of every grid square.
Check the MEF in every grid square along your entire route before selecting a cruising altitude. The highest MEF on the route sets your altitude floor.
Your chosen altitude must satisfy both the MEF clearance and the hemispherical rules under FAR 91.159 - these requirements do not automatically align.
For night cross-country flights, extend your MEF check 10–15 miles on either side of the planned route to maintain safety margin if you drift off course.
Sectional charts update every 56 days. Flying with a current chart is both a legal requirement and a direct safety issue for MEF accuracy.