22
a4’ – a1’ = a3’ – a2’
than a4’ = a1’ – a2’ + a3’
If the actual reading a4’ differs from the computed correct value a4 by more
than 2mm in 30m, the whole procedure must be repeated. If the difference is
confirmed, the line of sight must adjusted as described
Fig.6 Testing the horizontality of the line of sight
Fig.6
