Splitting Hairs!
Good point Daysleeper - and while we are on the subject (of splitting hairs) - the transmissometers (for there are normally three at any significant airfield) sit in the grass to one side of the r/w about 25m away from the c/l so they don't actually measure viz on the c/l. In shallow drifting fog this can be a right royal pain in the bum!
The original post/er seems a little confused/confusing..?
Care to expand upon the question?
OK - back in box...