1. Theodolite - Instrument Checks
There are 6 possible instrument errors:
1) Plate Bubble Error . This is checked every time the theodolite is set up.
i.e. check the bubble drift in positions c) and d) and then eliminate the error
by “ Freezing the Bubble”.
2) Non - Vertical Cross Hairs . Move the telescope up and down while
sighting a well defined distant point. Any error will be very obvious !
3) Horizontal Collimation Error . Sight a well defined point and read the
horizontal circle on F/L and F/R. The difference in the minutes and seconds
should be <40”
4) Trunnion Axis Dislevelment . - Spire Test. Sight a well defined elevated
point (church spire) then lower the telescope onto a tape or staff about 40m
away. The difference between F/L and F/R should be < 5mm for most
engineering applications.
It is not acceptable for PLUMBING PURPOSES e.g. verticality of columns.

2. Theodolite - Instrument Checks (cont)
5) Vertical Collimation Error . Sight a well defined point and read the
vertical circle on F/L and F/R. The difference in the minutes and seconds
should be <40”
(the vertical collimation error is often larger than this and for slope corrections
it is not too critical) e.g. a 60” error in an angle of 50 over a sloping distance
of 100m is given by :
Horizontal distance = 100 cos or cos ’ 00 ”
= or
i.e. an error of 2mm.
6) Optical Plumb Error . Check the optical plumb alignment in two positions
1800 different.
Rotated by 1800
Readjusted to mid point
Initial position