Merge activities into one file

A watch that dies partway through a long ride, a route recorded in two segments because you paused and restarted the app, a friend's file for the second half of a group ride you want folded into your own — all of these leave you with pieces of one activity spread across separate files, and stitching them back together by hand means opening each one and figuring out where they connect. This tool does that stitching automatically: drop in the files, and it orders them by their actual recorded start time rather than the order you happened to upload them in, so it doesn't matter if you drag the second half in before the first. Where it earns its keep is the case that trips up naive merging — overlapping time ranges, which happen more often than you'd expect when two devices recorded the same stretch of a route. Rather than silently picking one file's points and discarding the other's, or interleaving them in some arbitrary way that might duplicate or contradict a whole segment, it detects the overlap and shows it to you directly, then lets you choose how to resolve it: keep the earlier file's points for that window, keep the later file's, or interleave both by timestamp. Nothing is decided on your behalf without being surfaced first. Inputs don't need to match format either — a GPX export from one watch and a FIT file from another can be merged into a single output together, since the merge operates on the parsed activity data rather than the source file's binary or XML structure. As with every tool here, the files are read and combined entirely inside your browser; nothing is uploaded to make this work, which matters if the route in question passes anywhere private. Free for files up to 25 MB each, three per batch.