Airports, Towers and Obstacles on a Chart
Beyond the basic airport symbol shape and color covered in the previous lesson, a sectional chart packs a full data block of practical information next to every airport, and a precise two-number height convention next to every charted obstacle. This lesson teaches how to read both — skills a remote pilot uses on nearly every real flight, and a frequently tested piece of chart-reading knowledge.
The airport data block
Next to every charted airport symbol, a sectional chart prints a compact block of text data. This typically includes the airport's name, its identifier code (a short FAA or ICAO designator, often three or four letters or numbers), and — critically for altitude planning — the airport's field elevation in feet MSL, usually shown as a plain number placed near the symbol, sometimes boxed for emphasis at larger airports. Many airport data blocks also include the Common Traffic Advisory Frequency (CTAF) for non-towered airports or the control tower frequency for towered airports, along with an indicator for runway lighting availability, often a small "L" tag meaning the airport has runway lighting.
Airport elevation matters well beyond the airport boundary itself: since chart-published airspace altitudes and obstacle heights are given in MSL, knowing the ground elevation at or near a planned flight site is the starting point for converting any MSL figure into a usable AGL altitude for that specific flight — the full conversion process is covered in the next lesson.
Reading obstacle height: the MSL/AGL number pair
Every charted obstacle prints two height numbers together, and knowing which is which is essential. The number printed WITHOUT parentheses is the obstacle's height in feet MSL — its height above sea level, at the very top of the structure. The number printed IN parentheses is the same obstacle's height in feet AGL — how tall the structure itself is, measured from its own base. For example, an obstacle label reading "1071 (351)" means the top of that obstacle sits at 1,071 feet MSL, and the structure itself is 351 feet tall measured from the ground it stands on.
This AGL figure is exactly the number a remote pilot needs for two of the most common Part 107 planning checks: comparing an obstacle's height against the 400-foot AGL operating ceiling under 14 CFR 107.51, and determining whether the "structure exception" in that same rule — which allows flying up to 400 feet above a structure's immediate uppermost point when within 400 feet laterally of it — could apply to a nearby inspection job.
Obstacle symbol size and grouping
Obstacles that reach 1,000 feet AGL or taller are shown with a larger, bolder version of the standard obstacle symbol, making genuinely significant hazards easy to spot at a glance even on a busy chart. Where multiple obstacles are clustered closely together — a common pattern for guyed communication towers or wind farms — a single group obstacle symbol is used instead of plotting each one individually, with the printed height figures reflecting the tallest structure in that group. A pilot planning a flight near a group symbol should treat the entire clustered area as hazardous, not just the single charted point.
Real-world scenario: clearing a tower before an inspection flight
A pilot is hired to inspect solar panels on a warehouse roof and notices a charted obstacle nearby labeled "850 (410)." The 410-foot AGL figure immediately tells the pilot this tower exceeds the standard 400-foot AGL ceiling under 107.51 — meaning the pilot must either stay well clear of it, request a waiver, or confirm whether the limited structure exception genuinely applies to the specific inspection site (it does not apply to this unrelated tower, since the exception only covers flying near the structure actually being inspected). Reading the AGL figure directly off the chart, rather than trying to estimate the tower's height visually, is what makes this kind of planning decision reliable rather than guesswork.
An airport data block packs name, identifier, elevation, and frequency information next to the symbol itself.
View Larger
The unparenthesized number is MSL height; the parenthesized number is AGL height — the figure a pilot needs for 107.51 planning.
View Larger| Data element | Where shown | What it means | Notes |
|---|---|---|---|
| Airport identifier | Adjacent to airport symbol | Short FAA/ICAO airport code | Used to look up the airport in the Chart Supplement |
| Field elevation | Plain or boxed number near symbol | Ground elevation at the airport, feet MSL | Reference point for MSL/AGL conversion nearby |
| CTAF/tower frequency | In the data block | Radio frequency for that airport | Not typically used for routine Part 107 flights, but useful situational awareness |
| Obstacle height (no parentheses) | Next to obstacle symbol | Height above sea level (MSL) | Top of the structure, feet MSL |
| Obstacle height (in parentheses) | Next to obstacle symbol | Height above ground (AGL) | The figure most relevant to 107.51 planning |
How to read an obstacle's full data label
Applying this quickly and correctly is essential before any flight near a charted tower or structure.
Locate the obstacle symbol and its height label
Identify whether it's a single-obstacle symbol or a group symbol, and note the printed height figure(s).
Identify the MSL figure (no parentheses)
This is the obstacle's absolute height above sea level — useful for comparing against nearby airspace ceiling figures, which are also in MSL.
Identify the AGL figure (in parentheses)
This is the obstacle's actual height off the ground — the figure to compare directly against the 400-foot AGL operating ceiling under 14 CFR 107.51.
Check whether the structure exception could apply
If the flight is specifically inspecting that structure and stays within 400 feet laterally of it, the 107.51 structure exception may allow flying up to 400 feet above its immediate uppermost point — this does not apply to unrelated obstacles elsewhere on the chart.
| Mistake | Why it happens | Correct understanding | Regulation / source |
|---|---|---|---|
| Reversing which number is MSL and which is AGL. | Both numbers look similar without a clear memory aid. | The number in parentheses is always AGL; the number without parentheses is always MSL. | Sectional chart legend |
| Assuming an airport's field elevation applies uniformly to the whole surrounding area. | It's the only elevation figure readily visible near the airport. | Ground elevation can vary across a wider flight area — the airport figure is a reference point, not a universal value. | Sectional chart legend; next lesson (MSL vs AGL) |
| Applying the 107.51 structure exception to any nearby obstacle. | Confusing "near a tall structure" with "inspecting that specific structure." | The exception applies only to the structure actually being flown near/around for the operation, within 400 feet laterally of it. | 14 CFR 107.51 |
Which obstacle height number do I actually need for Part 107 planning?
Almost always the AGL figure — the number in parentheses — since 14 CFR 107.51's 400-foot ceiling and structure exception are both measured in AGL terms.
Do I need to check every single obstacle on a chart before flying?
Focus on obstacles near your actual flight path and altitude, especially any close to or exceeding 400 feet AGL. A full-chart obstacle inventory isn't necessary for a specific, localized flight.
How is airport elevation shown if it isn't boxed?
At smaller or less prominent airports, the elevation figure may simply be printed as plain text near the symbol rather than boxed — the boxing is a visual emphasis convention, not a requirement for the number's validity.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.