no sig... All the bloke in the tower does is to read the anemometer dial. Whether that's mag or true I don't know, although I've always assumed it to be mag. (I never saw any adjustments applied to the equipment when I worked in Heathrow Tower). I did a spell as a met observer yonks ago and my reports simply quoted the wind off the anemometer and I assume that's the info we all get in metars so precisely "which" wind it is I don't know.