Hydromet Stations Question

[Prairie Buoy]

Hi people. I have a question for those who may know... Just for my own interest I daily check the live video of the Hydromet station along the Sproat River. I like to see the socks swim by and later a few springs. Is impossible to gage fish size from watching; but I see there are a couple of vertical lines in the background behind the fish.... I assume these lines are to give some idea as to size... does anyone know what this distance between the lines is or where I can get info on??

Thanks
PB