Lesson 13: Obstacles, Alerts, and Maximum Elevation Figures

Welcome back, future pilot! You did a great job last week learning about how to calculate longitude and latitude. Today, we’re going to decode sectional charts some more. 

Did you know that sectional charts contain hundreds of unique symbols that show stuff from mountains and rivers to railroads and amphitheaters?

And each one can get even more specific:  Is the lake natural? Does it change? Is the sand really wet? How many tracks does a railroad have? There’s a symbol for that.  

National Park, Wildlife Refuge, Primitive and Wilderness Areas

High energy radiation areas 😱

Super populated big cities  

Time zones 

Dams

Airplane landing and refueling sites 

Single-track
Railroads 

Railroads under construction 

Power & transmission lines 

International boundaries 

Landmarks 

Coral or rocky
reefs 

Umm.. Intense, right? Well, do we have some good news for you. 

Luckily, you don’t need to know any of this stuff as a drone pilot! That’s for manned aircraft pilots to know about. That’s pretty relieving!   

Us drone pilots only have to know a few symbols. And even better news? 

The only symbols we need to know on a Sectional Chart can be found in The Legend! 

Legendary.  

The Legend on a sectional chart is a guide that explains what different icons mean to make it easier to navigate and plan flights.

The Legend on a sectional chart is a guide that explains what different icons mean to make it easier to navigate and plan flights.

The Legend on a sectional chart is a guide that explains what different icons mean to make it easier to navigate and plan flights.

Here’s what it looks like! 

Today, we’ll be covering one important area: obstacles (the FAA calls them obstructions).  

Hey! Does anyone know what an obstacle or obstruction is? 

Right! Obstacles and obstructions are things that get in the way of doing something. 

According to the FAA, an obstruction is anything that can be a hazard to aircraft, such as tall buildings, towers, or trees that are high enough to interfere with flight paths.

Obstruction

It’s important we know about obstructions so we don’t crash into them! So, let’s explore these obstacles and their symbols. 

Obstructions

Single Obstructions 

Group Obstructions 

High-intensity obstruction lights 

Single Obstructions 

These icons indicate single obstacles, like a tall TV and radio tower or even a huge monument or statue.  

Single Obstructions 

Ah… What are the numbers for?



Number on top → height in MSL

(Number) on bottom → height in AGL

MSL vs AGL Review 

MSL 

AGL 

Mean Sea Level (MSL) is the average height above standard sea level.

Above Ground Level (AGL) is the actual height above the ground you’re flying over. 

You might see an obstacle on a chart like a radio tower.

Next to the symbol for it, you might see “1049 (1036)”.

Guess what this means?

This means the radio tower is 1,049 ft MSL and 1,036 ft AGL. 

It’s exactly 1,036 ft away from the ground. The MSL height is different because of the elevation of the ground.

1,049 ft MSL

1,036 ft AGL

Remember–you can fly up to 400 feet above the topmost part of the tower. 

That is the highest altitude at which you are allowed to fly your drone.

400 feet

You should always keep an eye out for towers and other obstacles on a sectional chart. 

Look at what the height is in both MSL and AGL.

MSL vs. AGL 

Great work! Now, let’s cover the next kind of obstruction. 

Review

Group Obstructions 

These are basically groups of towers that are clustered together, like wind farms or high-rise building complexes.

Group Obstructions 

The numbers are the same!



Number on top → height in MSL

(Number) on bottom → height in AGL

High-intensity obstruction lights 

Very tall structures, like tall buildings, radio towers, and bridges usually have these bright flashing lights on them. 

These lights  help make the structures more visible to pilots at night, since they could be dangerous to aircraft. 

High-intensity obstruction lights 

If you see these electric icons on the obstruction symbols we just learned, just know to watch out for super
bright, flashing lights! 

P.S. – On a Sectional Chart, obstructions look like this. Can you spot them? 

Nice job! Let’s see how much we remember about the obstacles we just talked about.  

Obstructions

Which icon is a group obstruction? 

Obstructions

A

B

C

D

Which icon is a group obstruction? 

Obstructions

A

B

C

D

Which icon is a single obstruction? 

A

B

C

D

Obstructions

Which icon is a single obstruction? 

A

B

C

D

Obstructions

Which icon shows high-intensity lights? 

A

B

C

D

Obstructions

Which icon shows high-intensity lights? 

A

B

C

D

Obstructions

Woah, Nice Work! We’re feeling pretty proud. Keep it up!  

Great work! Now, let’s move onto our next subject: VFR Checkpoints.

Do you remember what ‘VFR’ stands for? 

What’s Up Ahead

VFR Checkpoints

Using landmarks to navigate 

Do you remember the difference between VFR vs. IFR?

VFR vs. IFR

VFR relies on visual references for navigation

IFR uses instruments and coordination with air traffic control to navigate through different weather conditions

Imagine you're driving a car on a road trip. You look for landmarks like big signs, tall buildings, or even cool statues to know where you are and where to turn next.

Pilots do something similar in the sky, but they use VFR checkpoints instead!

VFR checkpoints are places on the ground that are easy to spot from the air.

These could be things like big lakes, highways, tall towers, or even unique buildings.

Pilots look for these checkpoints to help them find their way and make sure they’re on the right track.

On a Sectional Chart, VFR checkpoints show up as magenta flags.

Can you spot it? 

Nice! That’s a VFR Checkpoint. 

Let’s define it! 

A landmark that helps airplane pilots know where they are when they are flying. 

As a drone pilot, all you need to know about them is that they are shown on Sectional Charts as a magenta flag, and that there might be lots of airplanes in those areas.

If you’re operating near an airport or leaving class G airspace, you need to know everything about the airspace around you.

A small magenta flag on a Sectional Chart indicates a Visual Reporting Checkpoint or waypoint for manned VFR aircraft.

You should expect more manned aircraft (think “airplanes”) in those areas. 

Keep this in mind when preparing for your flights.

Notification Boxes and MEFs 

How to stay aware, and decode the highest obstacles!

Did you know that there’s another detail on maps that helps pilots navigate safely? These are called notification boxes! 

And, they do what they sound like they do. 

They notify you about stuff happening in real time! Like, they might say “extensive flight testing” or “AHH!” there’s an “unmarked balloon!”

These boxes appear on charts as a rectangle with blue text. Make sure to read them carefully!

Notification boxes notify you about things happening in real time. 

For example, they might say “extensive flight testing”, or there’s an “unmarked balloon”.

TFRs and NOTAMs 

Temporary Flight Restrictions and Notices to Airmen / Air Missions

Speaking of notifications, let’s talk about two more ways that pilots can get alerts about stuff they should know about. 

Let’s learn how to navigate Temporary Flight Restrictions and Notices to Air Missions.

TFRs & NOTAMs!

We can start with Temporary Flight Restrictions
or TRFs for short. 

TRFs pop up when something unusual is happening in the sky, that has to restrict your flight there.

Think of times like when politicians are traveling, during big events, or during natural disasters.

The FAA when writing TFRs: 

In fact, let’s take a look at some current TFRs by visiting this government website that’s specially dedicated to TFRs! 

You’ll see that they cover things like hazards, shows and sports, firefighting, and security reasons! 

They’re common over stadiums, too (...imagine
if your drone failed mid-flight over one!) 

Click on any section under ‘description’ to find out more details about a TFR! What did you find? 

You should check TFRs before every flight, to make sure you don’t run into any unexpected detours.

TFR

A Temporary Flight Restriction (TFR) is like an invisible force field in the sky that tells pilots they can't fly in a certain area for a short time. 

It’s the FAA’s warning for the entire airspace! 

This could be because of something special happening there, like a VIP visit, a big event, or an emergency. 

It's like a "no-fly zone" that keeps all planes away for safety or security reasons – including your drone! 

You can remember TRFs as TURFs! Like someone saying “Hey! Stay off my turf!” if you fly there. 

This might be a little bit confusing, but TFRs are a type of NOTAM (Notice to Airmen or Air Missions).

NOTAMs means ‘Notices to Airmen’ or ‘Notices to Air Missions.’ That makes you an Airman – or, an Airwoman, or Airperson. Hehe.

Anyways, let’s not get too carried away, and explain what a NOTAM is! 

NOTAMs are messages that give pilots important time-critical information about airspace.

They can alert pilots about things like runway closures, flight restrictions, or special events that affect flying.

Some reasons for NOTAMs include: 

Runway closure due to snow, ice, or damage 

Workers doing maintenance near a runway 

A crane working on a building near the airport

Concerts, air shows, or military exercises 

And, you’d see it written this way:

“Runway 27 closed for repairs" or "Flight restrictions due to a concert."

The FAA requires all flight crews to review Notices to Air Missions (NOTAMs) before every flight for safety reasons. Then they fly around! 

And remember, NOTAMs can include information about airspace restrictions, like Temporary Flight Restrictions (TFRs), which limit aircraft operations in certain areas for a set period of time.

A Notice to Air Missions (NOTAM) is an advisory message that alerts pilots about potential hazards along their flight route, or at a location that could affect the flight.

Airplane  pilots read NOTAMs before every flight, as it’s required by law, and plan their flights with NOTAMs in mind.

NOTAMs help pilots stay informed and make safe decisions during their flights. This includes you! 

And remember! NOTAMs can include Temporary Flight Restriction (TFRs). 

NOTAMs help pilots stay informed and make safe decisions during their flights.

And remember, NOTAMs can include information about airspace restrictions, like Temporary Flight Restrictions (TFRs), which limit aircraft operations in certain areas for a set period of time.

Finally, let’s move onto the last topic of the day! Maximum Elevation Figures! 

It requires a little bit of math, but after learning about longitude and latitude and how to calculate floors and ceilings of airspace, this will be a piece of… upside down cake! 

Nice! So far, we learned how to read classes of airspace, special use airspace, obstacle markings, notification boxes, and how to use the legend on a sectional chart. 

The last two sectional chart markings we’ll learn about in class are Quadrants and Maximum Elevation Figures!

Great! We’ll need to remember Longitude and Latitude for our next keyword that shows us how the big chart is divided up. 

L&L Recap

Sectional Charts are divided into Quadrants. Quadrants are just squares. Each one represents one degree of Longitude and Latitude! 

They’re big squares with lines in them, and they’re all over the map. They’re hard to see, but can you find these quadrants? 

Quadrants

Fun Fact: We also sometimes call these squares a silly word: Quadrangles 😂😂😂

Hold on – A what? Lol

It’s time to peel the onion! 

Let’s explain Quadrants (also called Quadrangles!)

Each area on a map has its very own quadrant! 

Quadrant 

Sectional charts are divided into squares called quadrants. 

Each quadrant measures half a degree of latitude and half a degree of longitude. 

This helps pilots and navigators accurately pinpoint locations on the map.

And if you look closely, it gives you the Longitude and Latitude coordinates,
so pilots know where they are. 

Because quadrants show us only half a degree, that’s why we see the Latitude and Longitude numbers shown on every other quadrant. 

Remember minutes? 

There are 60 minutes in one degree! 

Minutes are a way to measure part of a degree in latitude and longitude more accurately. 

If a quadrant is half a degree, can you guess how many minutes there are? 

That’s right! 30 minutes. 

Each of these ticks shows one minute. There are 30 minutes in one quadrant.

When in doubt, just count!

Awesome! Makes sense, right? Next up, we’ll talk about the last symbol that we always see on charts that we should memorize as remote pilots! 

Have you ever noticed a big number next to a little number on a chart? 

Well, every single quadrant has one! Can you find some in the next image? 

Nice work! Those are Maximum Elevation Figures (MEFs). Let’s talk about them! 

MEFs

On a sectional chart, a Maximum Elevation Figure, or MEF, is shown
as a big number and a little number. 

Based on the name, can you guess what a ‘Maximum Elevation’ Figure is for? 

A Maximum Elevation Figure shows us the highest (a.k.a. maximum) elevation, including terrain and other obstacles like towers, within each quadrant.

It shows you the minimum altitude that you can fly at within one quadrant, so you can still be able to know and clear all the obstacles inside of it.

To know what the highest obstacle is, we have to do some calculations, though!  

But, you’ve done something similar before, so don’t be too worried!

(And that’s adding zeros!)

You got this.  

The big number is the height in thousands of feet (MSL), and the little number is in hundreds. Then, you add the zeros.

You can add 3 0’s to the big number, and add 2 0’s to the small number. 

This number is calculated by getting the height of the tallest obstacle, rounding it up. 

So the tallest obstacle for this example would be 2,600 feet MSL.

2,000 Feet + 600 Ft = 2,600 Ft MSL  

So, looking at this number, how tall is the highest obstacle in the quadrant?

3,400

3,500

34

35,000

4

3

That’s 3,000 feet + 400 ft.

= 3,400 feet! 

Great work! I see you’re all a few steps closer to becoming a certified drone pilot.