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IGC file format homepage

I've written a few different programs for creating and analysing the IGC-format log files used in soaring as the accepted transcript of the route of a flight, and this page collects reference information helpful to developers.

As a plea, if you read an IGC file into your software, just pick up the records and fields meaningful to you and ignore records and fields you don't care about. If you want but don't get a 'pilot date of birth' (this is an optional field in an IGC file, I kid you not) don't have your software crash... just continue with a default value. If you see a record in an IGC file you don't recognise, just skip it. 99.999% of what needs to be done with an IGC file comes from reading the timestamped 'B' tracklog records (time, lat, long, alt), everything else is pretty much irrelevant overhead for most purposes. 'B' records can have additional optional values tagged on the end (e.g. speed) so if you just need time, lat, long, alt please don't have your software crash because the 'B' record is slightly longer than you expected, just read the fields you want and ignore the rest of the line.

In summary, the IGC file format is like every other 'GPS trail' file format, in that it is basically a container for a series of plain-text time-stamped location records one-per-line giving:

To cut to the chase a real IGC file could be as below (although most software will require additional header records):

And to explain the basic record format, using commas to indicate the actual fields:

So if we call the 'B' record a 'trackpoint' record, you can see an IGC file without any would be fairly pointless (that's a pun, by the way). However, IGC files can contain quite a few other record types, each indicated by a unique first letter and similarly terminated by a newline. Nearly all of the additional records are placed at the beginning of the IGC file, recording the DATE the trackpoints were created (this is important as the B records only contain the TIME) and also the name of the user, registration of the aircraft involved, type of logger used, other general stuff of that nature. NONE of these records actually care about the content, e.g. what is written for the type of the logger, so if that record is irrelevant for your requirement the usual approach is to populate the record with something generic, rather than omit the record.

So an IGC file that pretty much all display software would be happy with could be:
AXXX sim_logger v2.31
HFDTE020911
HFFXA035
HFPLTPILOTINCHARGE: Ian Forster-Lewis
HFCM2CREW2: not recorded
HFGTYGLIDERTYPE:Standard Libelle 201 B
HFGIDGLIDERID:B21
HFDTM100GPSDATUM: WGS-1984
HFRFWFIRMWAREVERSION: 2.31
HFRHWHARDWAREVERSION: 2009
HFFTYFRTYPE: sim_logger by Ian Forster-Lewis
HFGPSGPS:Microsoft Flight Simulator
HFPRSPRESSALTSENSOR: Microsoft Flight Simulator
HFCIDCOMPETITIONID:B21
HFCCLCOMPETITIONCLASS:
B1140115249652N00212031WA0009600096027000
...

You would need to check the spec to understand the meaning of each of these header records, but the important thing to know is that the green stuff is whatever you want it to be. Software reading the file may or may not display or discard the HFGIDGLIDERID (the 7th record i this example), but if your usage doesn't actually have a glider id then set it to XX or anything else you don't care about...

In terms of priority, assuming you're coming to this page because you want to understand the IGC file format, you see from above you can get an effective tracklog file quickly with the header records plus a string of 'B' trackpoint records. Be prepared for most software to throw away most of your carefully considered header information, but you want to include it for other shite software out there which crashes if the file doesn't have e.g. a HFGIDGLIDERID: record.

• Task (Course or Route) definition: In addition to the critical B record, the IGC file format can contain a task (i.e. route or course) definition using a similar C record, so your IGC file can contain both the actual trail followed (in the B records) and the originally intended 'route'. This is particularly suitable for aviation as it is normal to declare your major waypoints in a flight plan before you take off.
• Additional fields on the tracklog B record can be defined with the header I record. So you are not limited to the default time, lat, long, alt set of fields. For example you could easily have your tracklog records be time, lat, long, alt, speed, heading. The defined additional fields are documented in para 7 of the spec. In practice these fields are almost always ignored by tracklog display software so actually they can be conveniently used for your purposes if necessary, e.g. if you wanted to store aircraft bank in a field you would make up your own code or hijack an existing one.
• Comment records: Finally a useful catch-all if you want to store your own proprietry information in the IGC file is the comment or L record. Many software developers (including me) use L records to embed their own data in the file. In my case for flight simulator use I can record the real PC date and time the file was saved, while the main IGC date and timestamps actually record date/time in the simulator. If you do use the L record, use a reasonably unique string immediately after the 'L' so your records don't confuse other software that might also be trying to pick out data from their own comments. E.g. here's a comment from the log files I generate in my Flight Simulator logger (my comments use 'L FSX'):
  L FSX user PC time 2011-10-01_0933

I re-structured the FAI IGC 'Appendix 1' document section detailing the file format into an HTML file with cross-referencing links within it, so that references to (for example) the crucial B record can be linked to the right part of the specification.

• IGC file format documentation in HTML with handy cross-reference links. I started with the 'Appendix 1' part of the PDF IGC specification and added all the cross-reference links. It took ages but I think the document is now more usable.
• Technical Specification for IGC-Approved GNSS Flight Recorders. The definitive document from the FAI. The HTML document linked above is a taken from Appendix 1. The FAI unfortunately rename links on their site at frequent intervals so links-in are almost impossible to rely on. If you can't find the file on the FAI site, use this copy on my site which I promise not to move around.
• IGC files can be directly uploaded to everytrail.com. This is a bit of an aside but damn fine, in my view.
• Here is a sample real IGC file (from an LX logger) which includes the engine firing up during the flight (see the 'engine noise level' in the last three characters of each 'B record' from 12:53:28 UTC).