22
Horizontal setting of the sight line
The sight line on the instrument must be horizontal within 3 mm to achieve sufficient measuring precision.
Set up and level the instrument on its tripod mid way between two levelling staffs.
The distance between the staffs should be about 30
–50 m. Centre the bubble in the spirit level.
Read off staff A, in this example 1.924 m.
Read off staff B, in this example 1.712 m.
H = A
– B = +0.212 m.
Point B is therefore 0.212 m higher than point A.
Move the levelling instrument and set it up about 1 m from point A. We know from the previous
measurement that B is 0.212 m higher than A. From the set up point to point A the height read off to B
should therefore be 0.212 m less than the height to A. Read off the staff at point A, in this example
1.696 m. The height read off to B should therefore be 1.696
– 0.212 = 1.484 m. Now read off the height
on the staff at point B. If the height read off is 1.484 ± 0.003 m, then the sight line is horizontal. The sight
line must be adjusted if the height read off deviates more than this.
Unscrew the viewfinder housing. Turn the adjuster
screw until the middle horizontal line in the cross hairs
is opposite the correct value of 1484 m on the levelling
staff at point B. Screw the viewfinder housing back on.
Do not overtighten the housing.
