Originally Posted by tennisplayer_85
Yes it is true that there is some human error and that this affects the accuracy, but itís not as bad as you think. Imagine you serve from the baseline to the service box line, which is exactly 60 feet.
Now you move the player 1 foot to the right because of human error.
If we use Pythagoras (a2 + b2 = c2) we can calculate the new distance 60(2) + 1(2) = 60.0083 feet.
A good formula is used to calculate at which height the player hits the ball. Imagine you hit the ball a foot higher than average. This means again that there is 0.0083 human error.
As you can see, moving the player or the ball horizontally and even the players height does almost not affect the speed at all.
Since the iPhone has a 60FPS camera the time factor is very accurate as well and almost does not give any room for error.
The only error is as you say, if the player hits the ball a foot inside the court, because this does affect the speed. The 60 feet distance would become 59 feet.
Which is 1.667% error. So if a player would serve 100 mph the serve would be 101,6 mph.
So letís say there is an error range about 2,5 miles or maybe a bit more but on average it wont be this bad. In my opinion this is very reasonable.
Also hold in mind that a speed gun (even the professional ones) have an error range too and need to be calibrated and are sometimes inaccurate.
Thanks for the reply. I think the bigger issue though is how u have to estimate the spot where the serve hit the court. I tried moving the spot around just a little bit and got readings as much as 10 mph apart. That's just a bit too large of a variance to make the app useful.