Temporary adjustment of a theodolite

Similarly to the levelling instrument we can distinguish between temporary and permanent adjustment of the instrument. The temporary adjustments have to be carried out every time the theodolite is used.

2.1 Setting up, centring, and levelling up.

Setting up and centring concern the procedure necessary to place the instrument vertically above the station to be occupied (stationpoint). Levelling up is to bring the vertical axis V truly vertical. Very often theodolite surveying involves measurement of angles from a certain station point. The instrument should then be centred vertically above the stationmark and at the same time accurately levelled up. A suitable procedure to achieve this situation can be described in 7 points:

1. Put the tripod above the indicated stationpoint as accurately as possible, and with the top of the tripod approximately horizontal. The tripod legs are preferably at equal distances from the station mark. Now put the legs firmaly into the ground, so that that will be no danger of distrubance during measurement.
2. Mount the theodolite on top of the tripod, making sure that the instrument is firmly attached to it.
3. Use the optical plummet to determine if the mark of the stationpoint is within view. If not, repeat from point 1.
4. Use the three footscrews to get the stationmark exactly in probably not horizontal in this position.
5. Adjust one or more tripod legs to level the theodolite approximately. The circular bubble is used for this ‘approximate levelling up’.
6. Check if the thodolite is still correctly centered above the stationpoint. Small deviation can be adjusted by loosening the screw with which the theodolite is attached to the tripod and shifting the instrument. If the deviations are too large repeat from point 4.

fig. 6 Approximate levelling up procedure.

7. Carry out the ‘accurate levelling up’ procedure of the theodolite (including centring above a stationmark). The plate level is used for this.

fig. 7 Accurate levelling up procedure.

a) Rotate the alidate until the plate level is parallel to tow of the footscrews, position (a). Bring the bubble to the center of its run using these footscrews.
b) Rotate alidade over 90o clockwise to position (b) and use the third footscrew to center the bubble again.
c) Rotate again over 90o to position (c), and determine whether the bubble remains central. If not, the plate level is out of adjustment and half the deviation has to be taken away with the footscrews, both in position (a) and (b).
d) If the whole procedure has been carried out correctly, then the bubble should remain in the same position while rotating the alidade over 360o.
e) Check if the theodolite is still centered above your stationmark. If not, repeat from point 6.

2.2 Removing parallax.

Necessary to bring images of object and cross hairs in the same optical plane, making accurate readings possible.

Turn the focussing sleeve to the infinity mark.
Point to the sky or hold a white sheet of paper in front of the telescope for even illumination.
Turn the adjustment screw at eyepiece end until the cross hairs appear very sharp and distinct. Since the value of the dioptric scale depends upon the characteristics of the observer’s eye it can be set every time the observer uses this instrument.