Every sectional aeronautical chart is a dense mosaic of symbols, colors, and numbers — all designed to give a pilot an accurate mental picture of the world below the flight path. For remote pilots operating under 14 CFR Part 107, reading those symbols correctly is not an abstract academic exercise. It is a core safety skill. Towers, antennas, power lines, and other man-made obstructions appear on sectional charts in carefully standardized ways, and the FAA knowledge test expects you to distinguish between them with confidence. This article breaks down exactly what those symbols mean, how to read the elevation data printed alongside them, and where the traps hide.
The authoritative source for sectional chart symbology is the FAA's Aeronautical Chart User's Guide, which is published by the FAA's Aeronautical Information Services and is available free of charge from the FAA website. The Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25) also covers chart reading in detail. Every remote pilot should keep both resources handy during ground study.
What Counts as an Obstacle
On a sectional chart, the term obstacle refers to any man-made structure tall enough to pose a hazard to low-altitude flight. The FAA requires that structures exceeding 200 feet above ground level (AGL) be reported to the FAA and depicted on aeronautical charts. Structures below 200 feet AGL may or may not appear, depending on whether they were reported and whether their position is considered a notable hazard. This cutoff is critical for Part 107 pilots: a drone operating under a standard 400-foot AGL ceiling can easily encounter obstacles that are not shown on the chart at all, especially in dense urban or industrial environments. Never assume chart absence equals obstacle absence.
The Core Obstacle Symbols
The FAA uses several distinct symbols to represent different types of obstacles. Understanding each one lets you quickly assess what you are looking at before a flight.
- Single obstacle (isolated): Depicted as a small symbol resembling a tower or spike, with a dot at its base. When the obstacle stands alone, the symbol is drawn without any grouping marks.
- Group of obstacles: When multiple obstacles cluster together — such as a wind farm or a row of transmission towers — they are shown with a symbol that has multiple spikes radiating from a shared base. This signals that you are not dealing with a single point hazard but a field of hazards spread across an area.
- High-intensity obstacle (UC symbol or flashing lights indicated): Some obstacles are equipped with high-intensity strobe lighting. On the chart, these are marked with a dot or asterisk-like enhancement near the symbol. Lighted obstacles are also noted with a small dot next to the obstruction symbol, while unlighted obstacles carry no such mark.
- Under construction (UC): Structures being built may be charted with the notation UC alongside the symbol. These may not yet be at their final height, so treat them conservatively.
Beyond these, you will encounter symbols for transmission lines and power lines, which are drawn as a dashed or dotted line between two tower symbols. These are especially dangerous at low altitudes because the wires themselves are nearly invisible and may span wide valleys or river crossings at heights well below typical manned aircraft traffic but directly in the path of a drone flight.
Reading the Elevation Numbers
Next to every obstacle symbol you will find two numbers stacked vertically, and reading them correctly is one of the most commonly tested skills on the Part 107 knowledge exam.
- Top number — MSL elevation: The number printed in bold on top is the obstacle's elevation above mean sea level (MSL). This is the absolute altitude of the obstacle's highest point, referenced to sea level.
- Bottom number — AGL height: The number printed in parentheses or in lighter type below the MSL figure is the structure's height above ground level (AGL). This tells you how tall the structure actually is relative to the terrain immediately around its base.
For example, you might see 1,549 on top and (499) below. This means the top of the tower sits 1,549 feet above mean sea level, and it rises 499 feet above the local ground. If you are flying in that area, you would need to remain below the obstacle's MSL elevation to avoid it — but you also need to know your own aircraft's altitude reference. Most drone pilots use AGL height operationally, but understanding the MSL figure lets you cross-reference with airspace floors that are always expressed in MSL.
A critical application: Part 107.51 establishes a 400-foot AGL maximum altitude for routine small UAS operations, with one exception under 107.51(b)(2) — when operating within a 400-foot radius of a structure, a remote pilot may exceed 400 feet AGL by up to 400 feet above the structure's immediate uppermost limit, provided the small unmanned aircraft remains within 400 feet of the structure. This means a 499-foot AGL tower creates a zone where you could legally fly as high as 899 feet AGL (400 + 499) within 400 feet of it horizontally — this exception exists specifically to allow inspection and similar work near structures, not as a general allowance around every obstacle. The chart's AGL figure is precisely what you need to calculate that ceiling correctly.
Lighting Designations
The FAA requires that structures over 200 feet AGL be lighted in accordance with FAA Advisory Circular guidance and FCC requirements. On sectional charts, a lighted obstacle is indicated by a small dot or flash-like mark near the obstruction symbol. An unlighted obstacle has the symbol alone, with no additional lighting mark. For night operations — which require a specific waiver under Part 107 — knowing which obstacles are lighted and which are dark is directly relevant to route planning. An unlighted 300-foot tower on a moonless night is essentially invisible and represents a severe collision risk.
High-Obstruction Areas and Maximum Elevation Figures
Sectional charts are divided into quadrangles, and each quadrangle displays a large bold number called the Maximum Elevation Figure (MEF). This number, printed in blue in the center of each quadrant, represents the highest elevation within that quadrant — including terrain AND obstacles. The MEF adds a buffer of 100 feet above the highest known feature when that feature (terrain or obstacle) is below 200 feet AGL, or a 200-foot buffer when the feature is 200 feet AGL or higher, and the result is then rounded up to the next higher 100-foot increment. The MEF is expressed in hundreds of feet MSL. So a figure of 28 means 2,800 feet MSL is the maximum elevation in that quadrangle. For Part 107 planning, the MEF is a quick sanity check: if the MEF for your flying area is unexpectedly high, that suggests prominent terrain or tall man-made structures worth investigating before flight.
Why This Matters for Part 107 Pilots
Manned aircraft pilots typically fly high enough that individual towers are minor concerns compared to terrain or weather. Remote pilots operate in exactly the airspace where towers, antennas, power lines, and construction cranes are concentrated — the low-altitude environment from ground level up to 400 feet AGL. The consequences of misreading an obstacle symbol are immediate and severe: a collision at drone speeds can destroy the UAS, create falling debris hazard for people below, and — in the worst case — strike a person or damage property.
Beyond physical danger, operating in the vicinity of obstacles often correlates with operating near controlled or special-use airspace. Communication towers, for instance, frequently sit on elevated terrain near airports. The discipline of reading obstacle symbols carefully trains your eye to read the entire chart carefully, including airspace boundaries, which are the other half of legal Part 107 compliance.
Key Numbers and Rules
- 200 feet AGL: General threshold above which structures must be reported to the FAA and depicted on charts.
- Top number = MSL elevation of the obstacle's highest point.
- Bottom number (in parentheses) = AGL height of the structure above local ground.
- 400 feet AGL: Standard Part 107 operating ceiling for small UAS.
- 400-foot radius exception: Under 107.51(b)(2), within 400 feet horizontally of a structure, you may exceed 400 feet AGL by up to 400 feet above the structure's immediate uppermost limit.
- MEF: Maximum Elevation Figure in each chart quadrangle; expressed in hundreds of feet MSL; includes a 100-foot buffer above features below 200 feet AGL, or a 200-foot buffer above features at or above 200 feet AGL, rounded up to the next 100-foot increment.
- Lighted obstacles: Marked with a dot or enhancement near the symbol; unlighted obstacles carry the symbol alone.
Common Test Traps
- Confusing MSL and AGL: The top number is MSL, the bottom is AGL. The exam frequently asks which number represents height above the ground, and many students invert them under pressure.
- Assuming unlisted = absent: Obstacles below 200 feet may not appear on the chart at all. The chart is not a comprehensive inventory of every hazard — especially in industrial zones.
- Misapplying the 400-foot structure exception: The 400-foot bonus altitude only applies within 400 feet horizontally of the structure. Beyond that radius, the standard 400-foot AGL ceiling applies immediately.
- Ignoring the MEF: Students often overlook the Maximum Elevation Figure entirely. Exam questions may describe an MEF value and ask what it represents or whether a given altitude is safe relative to it.
- Missing group obstacle symbols: A group obstacle symbol represents multiple structures spread across an area, not a single point. Flying the midpoint between two visible towers does not guarantee clearance from the entire group.