Identifying Airspace Boundaries on a Sectional Chart
Module 3 taught what each airspace class means and what a Part 107 pilot must do before flying in it. This lesson connects that knowledge directly to the chart: every airspace class has its own distinct boundary line style, color, and altitude notation on a sectional chart, and being able to read them quickly and correctly is a core, directly-tested skill on the Part 107 Knowledge Test.
Class B, C, and D: solid and dashed boundary lines
Class B airspace is drawn with a solid dark blue line. Class C airspace is drawn with a solid magenta line. Class D airspace is drawn with a dashed blue line. All three lines trace the lateral (horizontal) boundary of that airspace as seen from directly above — since Class B and C airspace typically consist of multiple stacked rings ("shelves") at different altitudes, a chart may show more than one concentric line for the same airport, each ring representing a different altitude segment of that airspace.
Because Class B and C are drawn as solid lines and Class D as a dashed line, the line style alone is often enough to tell them apart even before checking the color, which matters since color can be harder to distinguish quickly on some screens or in low light. Class A airspace, by contrast, has no boundary line at all on a sectional chart — since it's a uniform layer at 18,000 feet MSL and above nationwide, there is nothing localized to depict.
Class E airspace: three different chart depictions for one class
Class E airspace is the most visually varied of all the classes on a chart, because it appears in three genuinely different forms depending on where its floor sits. A Class E surface area — Class E airspace that extends down to the ground around an airport, functioning much like Class D for authorization purposes — is shown with a dashed magenta line, visually similar to (but a different color than) the Class D dashed blue line.
Where Class E airspace begins at 700 feet AGL rather than the surface, the chart shows a shaded magenta band that fades outward from the boundary — often called a "vignette." Where Class E begins at 1,200 feet AGL, the same fading effect is shown in blue instead of magenta. In both cases, the shaded edge marks the precise location of the floor's lateral boundary — the fading visual style is just how the FAA depicts this type of edge, not an indication of a gradual or uncertain transition.
Class G: identified by what's absent, not a line of its own
Class G, uncontrolled airspace, has no dedicated boundary line or color on a sectional chart — it exists by default anywhere none of the controlled classes (B, C, D, or E) reach down to the surface. In most of the country, Class G exists as a thin layer from the surface up to the base of the overlying Class E airspace — 700 or 1,200 feet AGL, depending on the area, as shown by the presence or absence of a shaded vignette. In sparsely populated areas of the western United States with no Class E designated at all, Class G can extend much higher, up to 14,500 feet MSL, before Class E's default floor begins. Identifying Class G on a chart is a subtraction exercise: rule out B, C, D, and E, and whatever airspace is left below the lowest controlled floor at that location is Class G.
Reading altitude figures: shelf fractions and boxed ceilings
Class B and C airspace print their ceiling and floor altitudes as a fraction-style number pair within each ring segment — for example, "70/30" printed inside a Class B shelf means that segment's ceiling is 7,000 feet MSL and its floor is 3,000 feet MSL. Where the floor is the surface itself, "SFC" is printed instead of a number, as in "70/SFC." All of these figures are in hundreds of feet MSL (mean sea level), not AGL — a critical distinction covered in full in the next lesson.
Class D airspace prints only a single ceiling altitude, inside a small boxed figure near the airport, since its floor is always the surface and does not need to be separately labeled — for example, a box reading "[25]" means a ceiling of 2,500 feet MSL. Some Class D and Class E surface areas operate only part-time, tied to when the airport's control tower is staffed; when the tower is closed, the airspace typically reverts to Class E or Class G as documented in the FAA Chart Supplement, a detail the chart symbol alone does not fully convey.
Real-world scenario: identifying airspace before an inspection flight
A pilot is hired to inspect a cell tower and pulls up the sectional chart for the site. The location falls just inside a shaded blue vignette with no solid or dashed line nearby — indicating Class E airspace with a 1,200-foot AGL floor, not a surface-level class requiring prior authorization. Below 1,200 feet AGL at that exact point, the airspace is Class G, and the planned 300-foot AGL inspection altitude requires no 14 CFR 107.41 authorization for airspace reasons. Had the chart instead shown a dashed magenta line enclosing that same point, the pilot would know it sits inside a Class E surface area and would need authorization before flying at any altitude there.
A composite practice excerpt combining multiple airspace classes, the kind of chart reading tested on the Knowledge Test.
View Larger
Class B/C altitudes print as a ceiling/floor fraction; Class D prints only a boxed ceiling figure since its floor is always the surface.
View Larger| Airspace class | Chart depiction | Altitude notation | Floor |
|---|---|---|---|
| Class A | No boundary line | Not applicable — uniform nationwide layer | 18,000 ft MSL |
| Class B | Solid dark blue line | Fraction, e.g. "70/30" (hundreds of feet MSL) | Varies by ring/shelf |
| Class C | Solid magenta line | Fraction, e.g. "40/SFC" | Varies by ring/shelf |
| Class D | Dashed blue line | Boxed ceiling only, e.g. "[25]" | Surface (always) |
| Class E (surface area) | Dashed magenta line | Generally not separately printed | Surface |
| Class E (700 ft floor) | Shaded magenta vignette | Implied by vignette color | 700 ft AGL |
| Class E (1,200 ft floor) | Shaded blue vignette | Implied by vignette color | 1,200 ft AGL |
| Class G | No dedicated line — identified by absence of B/C/D/E | Not applicable | Surface up to the overlying floor |
How to determine the airspace class at a specific chart location
Work through these checks in order — the first one that matches is the answer.
Locate the exact point on the chart
Pinpoint the specific location — airspace class can change over a short lateral distance, so precision matters.
Check for a solid boundary line first
A solid dark blue line means Class B; a solid magenta line means Class C. Note the shelf fraction if the point falls inside a ring.
Check for a dashed boundary line
A dashed blue line means Class D; a dashed magenta line means a Class E surface area. Both extend to the surface.
Check for shaded vignette shading
A magenta vignette means Class E begins at 700 feet AGL at that point; a blue vignette means Class E begins at 1,200 feet AGL.
If none of the above apply, it's Class G below the overlying floor
No line, no shading — the airspace from the surface up to whichever floor applies (700 ft, 1,200 ft, or 14,500 ft MSL where no Class E is designated) is Class G.
| Mistake | Why it happens | Correct understanding | Regulation / source |
|---|---|---|---|
| Reading Class B/C/D altitude figures as AGL. | AGL is the more commonly used reference in day-to-day Part 107 operations. | All charted airspace altitude figures are in MSL (hundreds of feet), not AGL. | Sectional chart legend |
| Treating a shaded vignette edge as a "soft" or gradual boundary. | The fading visual style looks less definite than a solid line. | The edge where shading begins is the exact lateral boundary — just as precise as a solid or dashed line. | Sectional chart legend |
| Assuming a location with no boundary line or shading has no airspace restrictions at all. | The absence of markings looks like the absence of any rule. | That location is Class G, uncontrolled airspace — a real airspace classification with its own (minimal) rules, not "no airspace." | 14 CFR 91.126–91.135; Module 3 |
| Assuming Class D always shows both a ceiling and floor figure. | Class B and C both print ceiling/floor fractions. | Class D's floor is always the surface, so only a single boxed ceiling figure is printed. | Sectional chart legend |
How do I know if Class E airspace starts at 700 or 1,200 feet without confusing the vignette colors?
Magenta shading means the 700-foot floor; blue shading means the 1,200-foot floor. If it helps, remember that magenta (the same color as Class C) marks the lower, "closer to the ground" floor of the two.
What if I don't see any boundary lines or shading at all at my location?
That means Class G airspace extends from the surface up to whatever floor is charted above it — 700 feet, 1,200 feet, or, in areas with no Class E designated, up to 14,500 feet MSL.
Are the altitude figures printed on a chart MSL or AGL?
Always MSL (mean sea level), given in hundreds of feet. Converting a charted MSL ceiling into a usable AGL altitude for a specific flight is covered in the next lesson.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.