hpurey:
2 feet I believe.
**edit**
this was quite ez to confirm.
Nah - you show 1.5 grids per 2 feet.
I found about 6 grids for 13', 4.5 for 9, 9+ for 20, on RSG #11.
Thus, it may be about 2' per grid but not an exact measurement.
Plus, the grid never crosses the hole AFAIR which is inevitable with an invariable distance.