I ran into an interesting difference between how alphas are calculate in GPSResults and GPS Action Replay. It appears that GPS Action replay always takes a run of 500 meters (or the closest it can get), while GPSResults will also consider runs - even much shorter runs, such as 100 m.

Most days, that difference does not matter, since the full 500 m runs will be fastest, and therefore selected by both programs. But just 3 days ago, I had GPS Action Replay give me a top alpha of 7 knots, while GPSResults gave more than 20 knots (details at boardsurfr.blogspot.com/2016/12/never-trust-software.html).

Looking at the GPS-speedsurfing.com and GPSTC web sites (respectively Hardie's explanation of alpha on this forum from 2007), it seems that the two sites use different definitions. GPSTC uses " total distance covered that is less than or equal to the maximum distance", while GPS3 uses "one run of 500 meters" (without the "less than"). Apparently, GPSResults implements the first definition, while GPSAR implements the second.

Since both GPSTC and GPS3 allow data analysis with either program, it seems that the difference in definition was not intentional (and perhaps even not noticed). It would be nice to hear from the respective "authorities" what the rule should be...