The instruments i use for testing the accuracy are videos from the australian open where the professional radar gun shows the speed in the end. I currently only tested 9 videos and these are the test results:
The results are from ServeSpeed V1.2 (in a few days available in the App Store)
Serves from the Australian open 2013
Video quality 1280x720, 59.94fps:
1st serve David Ferrer, Professional radar gun 197Km/h, ServeSpeed App 197Km/h (+/- 0)
1st serve David Ferrer, Professional radar gun 183Km/h, ServeSpeed App 182Km/h (+/- 1)
1st serve David Ferrer, Professional radar gun 190Km/h, ServeSpeed App 186Km/h (+/- 4)
2nd serve Novak Djokovic, Professional radar gun 156Km/h, ServeSpeed App 158Km/h (+/- 2)
2nd serve David Ferrer, Professional radar gun 149Km/h, ServeSpeed App 147Km/h (+/- 2)
Video quality 1280x720, 50fps:
1st serve Victoria Azarenka, Professional radar gun 152Km/h, ServeSpeed App 149Km/h (+/- 3)
1st serve Victoria Azarenka, Professional radar gun 159Km/h, ServeSpeed App 156Km/h (+/- 3)
1st serve Li Na, Professional radar gun 164Km/h, ServeSpeed App 160Km/h (+/- 4)
1st serve Li Na, Professional radar gun 161Km/h, ServeSpeed App 153Km/h (+/-
Average precision +/- = (27/9) = 3 Km/h = 1.86 miles
Hold in account that the last 4 videos are only 50fps.
Some videos are harder to calculate than others depending on the frame right at the point of contact. This is why the slider bar in the app comes in handy. This way its possible to say that the serve is in between 2 frames (making it more precise). At the beginning this might be a bit difficult but once you get the hang of it, it becomes easier.
That's why i made an extra video explaining how to get the best precision (so you get all the ins and outs). If you sign up for the mailinglist (through the website or from within the app) you will receive it right away. Also i'll keep you updated on upcoming updates like the sound detection etc!
These are real test results, people can try it themselves.
PS: I am planning to test many more videos and based on these statistic try to improve the accuracy in upcoming updates.