Where should start/finish mats be placed?

The start mat must be behind the start line, with the front edge of the mat on the line . Any backup mat must be behind that. The finish mat must be behind (past) the finish line, any backup mat must be behind (further past) the finish line. The mats must be repositioned after the race starts.

We use MyLaps equipment, their manual explains this.

This works easily, except for the case where the start/finish line is in the same place and the direction of travel is also the same, as in a loop course. Here you should measure the course with a gap between the start and finish lines equal to the distance between the two mats plus the width of the mats.

Thanks for the quick answers! It makes sense to me.

In the case you are talking about, Paul, a measurer would have to know the particular specs for the timing system that will be used-- right?

One more question, (to Guido Bro) if there is a backup mat at the start, where do you put that? And in all these schemes, do the runners line up behind the mat or mats? (as opposed to lining up right on the start line?)

The backup start mat should be placed such that the runners cross it before crossing the main mat when starting the race. The runners can stand on the mats. The runners form up at the starting line as if there were no mats. The timing equipment figures out which of the many signals received from each runner (on the mats) is the actual start time based on the time the gun goes off. This is entered by the timer.

For races with a coincident start- finish line where the runners start and finish in the same direction the mats can be placed with the front edge of the main mat on the start line, the backup mat behind it (runners cross the backup first). After the start the mats must be moved such that the edge of the main mat is on the finish line, the rest of the main mat is past the line. The finish backup mat is placed further past the finish line such that the runners cross it 2nd.

Let me simplify this:
Start
All timing mats/antenna must be behind the starting line (primary and back up). As previously mentioned the software will take the last transponder read - nearest to the starting line.
Finish
All timing mats/antenna must be beyond the finish.

Some timers still don't understand this concept but as a measurer it is very clear. If the start mats are place after the starting line there is always the chance that the transponder will be read on a short course!

I am grateful for all these responses including David's succinct summary. I will pass this information along to organizers and timers, and I would recommend that other measurers do that as well. The SCPF isn't meant to take care of incorrect mat placement!

For ChronoTrack systems, D tag use is fading in favor of B tags. I was told by one timer that now most of the reads are from the powerful side antennas (FlashPoints) vs low power mat antennas. What is the experience with accuracy and advice for positioning these? Does anybody know if the FlashPoints are linear or circular? There is not much information on the antenna itself.