the fence
The airport perimeter fence, and by extension the standard spotting position outside it.
Also written
fence linefenceline
"At the fence" means shooting from public land at the boundary, which is where most spotting happens and where most photographs on any hub were taken. Some airports build proper viewing areas along it; others simply tolerate it. Either way, the fence is the line — being on the far side of it is trespassing on an active airfield.