The area round the station may be divided into an Inner Zone and an Outer Zone. The inner zone is an annulus with the station at its centre and of radii from 1 to 2 km, while the outer zone is an annulus concentric with the first, of radii from 2 to 10 km. There is an additional Emergency Zone which com­prises an annulus of radii from 10 to 15 km. These zones are illustrated in Fig 4.10.

If this whole area around the station is further divided into quadrants, then there are three fixed

measurement sites per quadrant in the land sector of each zone so far as is geographically possible. Currently, routine measurements are made at each site in the inner and outer zones every three months, and at each emergency site every twelve months.