I think that the good chair umpires will always find the correct mark, if there is one, except maybe when there are so many identical marks in the area near the real mark that it's really difficult. It's hard to tell, I'm sure it's often very difficult and that the wrong ball mark is inspected sometimes, too.
But IF the real ball mark is found, I'm 100% sure that this mark is more trustful than what an automatic computer-based system is saying. These systems all use sensors to determine the slightly approximate position of the ball at various times and then calculate the trajectory from this data and where the ball finally lands. Because the ball mark of shot spot or such systems is "only" a calculation (quite a very good one, though, but not infallible) based on the trajectory details which involve a lot of unknown forces which depend again on other factors (resistent force of the air; buoyant force, spin and magnus effect...) or even hardly unknown ones (wind, IMO the biggest problem), the system can fail. I'm convinced it's still better and more reliable than the human eye of line judges or an umpire sitting 10m away (and therefore I welcome the electronic line calling on non-claycourts), but a proper ball mark is a proper ball mark with evidence on the court and so on claycourts I trust a real ballmark more than the current computer systems.
Originally Posted by almouchie
I have another question
other than the mark of the ball
well somehow related
what is the rule for sweeping the clay court
every set or 2 or what
& why dont they do that every set or every hour
its makes it easier to find the marks
& even better for the players
& the fans watchiing
it dosent take a couple of minutes even less
got be down at change over
Somebody should correct me, but I think there is no rule about this or an official section somewhere in the ATP rules. If you ask me, the common procedure usually is that it is done at the end of each set. But I've seen it often already that courts were swept at the middle of the set if it couldn't be done at the end of the set (because of any reason: for example if the crew doing this was occupied with something else at another court).