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Looking at the speed categories on the GPSTC website...peak speeds, nautical miles and one hour averages are all pretty self explanatory but what is "Alpha Racing 500m"?
Its basically 250m sailing, gype, 250m back, must be within 50m of starting point.........
Yes the speed is the average speed over the points measured. It can be any distance less than 500m.
But note that because the Gybe is the slowest past of the track, the more straight line reaching you do at higher speed, the faster the average will be. So in practice, the fastest Alpha 500's will always be as close to 500m as possible.
If you calculate the Alpha 1000m and Alpha 250m from the same track segment, (assuming that you continued long enough to go 1000m at approximately the same speed), you will almost invariably find the Alpha 1000 faster and the Alpha 250 slower.
So I can say truthfully that I have done a 30 knot Alpha! But it was an Alpha 1000!
For those who don't know, the speed of the Alpha is measured with the Doppler data, but the proximity circle is measured using the Locational data (trackpoint).
This is one of the reasons why Alpha speeds are not considered accurate enough for official records with the current technology. Better locational (and Doppler speed) accuracy from future GPS devices with higher hz data may change this.
The 500 is a maximum.
On ka72, until recently, we had a minimum distance based on 2x the radius, so your Alpha was the best gybe based on travelling between minimum of 100m and maximum of 500m.
Some "modern" GPS devices cause kind of a speed distortion when gybing blowing out the distance calculation (mainly affects devices that output FIT files) so I've decided recently to increase the minimum distance travelled, because some awfully short gybes were being detected as valid alphas. I think the new ka72 standard is 2.2x radius for the minimum distance travelled.
The Alpha is the most painful division to get right. Your inbound and exit tracks could pass within 50m of each other, but you might not get an alpha because there are no actual trackpoints that are within 50m of each other. Sometimes you get lucky, sometimes you don't. I have a feeling that some software works with this, but I could never be bothered. Alphas are slow enough to work out already.
(In the WindWanderers online speed comp, there is a 5m Alpha challenge division. You have to start and finish 5m apart. Otherwise the distance is still 500m maximum. That is more of a classic "Alpha" shape.)
But it was an Alpha 1000! 
)
most software picks the fastest part of the tracks that fall within the criteria. so if one leg is faster than the other, and the legs are under 50m apart it will use more of the fast track up to the 50m apart and under 500m total.
