Convert GPX to TCX

A GPX file has no concept of laps at all — no matter how many times you pressed the lap button mid-workout, GPX simply cannot record it, and a second trkseg element only marks a pause in recording, not an intentional split. TCX is different: it is Garmin's own training-log format, built around an activity broken into laps, each with its own summary stats. Converting to TCX does not invent lap boundaries that were never there — if your source GPX has no laps, the output TCX has one lap spanning the whole activity — but it does give you a container that training-log software actually understands, and any laps present are ones your file already carried through other means. There's a real quality gain here too: GPX only ever stores power through a third-party extension, a format nobody standardized on, so cross-device support for that field is spotty. TCX has power as a native element, so once the conversion is done, that channel reads back cleanly wherever TCX is accepted, including Garmin Connect. Garmin Connect and most training-log validators are strict about element order inside a TCX file, and this converter writes to that exact schema so the result opens without a validation error. Position, elevation, timestamp, heart rate, and cadence carry over exactly; only temperature has no home in TCX, so it's dropped if your GPX happened to have it. The whole conversion happens locally, in a background worker in your browser — the file never touches a server, so a private training route stays private, even for a multi-hour file with hundreds of thousands of trackpoints.