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@Greenbank
There are plenty of other ways of doctoring GPS tracklogs so that they don't exhibit this behaviour. Removing points and shuffling up timestamps (which is what digital epo did) is a very naive/basic method.
A while back I wrote some tools to analyse tracklogs to pick out the obvious ways of speeding up tracklogs (point removal/shuffling, interpolation, etc). This can be made much easier if there's extra data such as HR, cadence or power as it is much harder (but not impossible) to fake the speed increase along with realistic corresponding changes in HR/cadence/etc.
For example, if cadence data is present then can even be possible to have a reasonable guess at the gearing setup of the bike if you plot cadence against computed speed and look at the resulting bands that the points will fall into (albeit quite noisily).
I'm sure Strava can do this given their huge corpus of data. It'd be nice if they did this at some point in the future and retrospective flagged all of the dodgy looking uploads.
I'd love to see a GPS device that cryptographically signed each trackpoint (just the time and position data is enough) so that they could not be messed with, but there's no chance of this.