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.

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 50 or 100 cos 50 01’ 00 ” = 99.619 or 99.617 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