The OpenAir airspace format, made legible.

Each airspace is a block of records that begins with an AC (airspace class) record and runs until the next AC or end of file. A block carries metadata records — name, ceiling, floor, frequency — and one or more geometry records that trace its horizontal boundary.

Two dialects coexist. v1 (original / Winpilot) defines the core airspace and terrain records. v2 / v2.1 (extended / Naviter) adds structured metadata — airspace type, frequency, station, activation times, transponder code — and a version header. v2.1 is the current, actively maintained dialect.

Plain text, one record per line, UTF-8. A record is a 1–2 letter prefix, a space, then its argument(s). Both CRLF and LF line endings should be tolerated.

Header (v2+)

*VERSION: 2.1
*WRITTEN_BY: We-Fly

The * prefix marks a comment to end of line; v1 parsers ignore the header harmlessly.

Coordinates are written latitude longitude, each with a hemisphere letter, in WGS84. Both notations below must be accepted; the hemisphere letter may be attached or space-separated.

A value is a number plus a reference datum, or a symbolic keyword. Values are in feet unless stated. A trailing ft is optional and inconsistently present.

AMSL (Naviter/openAIP) and MSL (Winpilot) are equivalent — accept both. Examples: AH FL145, AH UNL, AL 1000ft AMSL, AL GND.

Filter by name or code, or narrow by category. Categories mirror an aeronautical chart legend — colour encodes what a record does.

The AC record carries an ICAO class letter or a legacy token. In the extended dialect, structural nature (CTR, TMA, danger…) moves to AY, leaving AC for the ICAO class — but older files overload AC. Accept both.

The AY record gives the structural airspace type, complementing the ICAO class in AC. Known values:

Edit the file on the left. Every line is parsed live — the record code is colour-coded by category, coordinates are converted to decimal, and altitudes are spelled out. Unknown lines are flagged.

• Block delimiters. Rely only on AC as the boundary. Never require blank lines; never treat comments as separators.
• Arc winding. DA/DB depend on the current V D=. Reset direction before each arc; a missing V D= means clockwise (+).
• Center lifetime. V X= persists until reassigned. A circle/arc with no preceding V X= is malformed — reject, don't guess.
• Radius units. DC/DA radii and V W= width are all nautical miles, unit omitted.
• Altitude synonyms. Accept AMSL≡MSL, GND≡SFC, optional ft; UNL only as a ceiling.
• Coordinate notation. Support DMS and DDM, hemisphere attached or detached.
• Unknown records. Skip unrecognised prefixes rather than failing the whole file.
• Polygon closure. Polygons are implicitly closed; don't assume the last DP repeats the first.