In order to maintain the primary axes of the theodolite in their correct geometrical relationship (Figure 5.3), the instrument should be regularly tested and adjusted. Although the majority of the resultant errors are minimized by double face procedures, this does not apply to plate bubble error. Also, many operations in engineering surveying are carried out on a single face of the instrument, and hence regular checking is
Vertical circle index error. (a) Face left, (b) face right
Tests and adjustments:
(1) Plate level test:
The instrument axis must be truly vertical when the plate bubble is centralized. The vertical axis of the instrument is perpendicular to the horizontal plate which carries the plate bubble. Thus to ensure that the vertical axis of the instrument is truly vertical, as defined by the bubble, it is necessary to align the bubble axis parallel to the horizontal plate.
Test: Assume the bubble is not parallel to the horizontal plate but is in error by angle e. It is set parallel to a pair of footscrews, levelled approximately, then turned through 90◦ and levelled again using the third footscrew only. It is now returned to its former position, accurately levelled using the pair of footscrews,
and will appear as in Figure . The instrument is now turned through 180◦ and will appear as in Figure 5.22(b), i.e. the bubble will move off centre by an amount representing twice the error in the instrument (2e). Adjustment: The bubble is brought half-way back to the centre using the pair of footscrews which are turned by a strictly equal and opposite amount. The bubble moves in the direction of the left thumb. See Figure 5.28. This will cause the instrument axis to move through e, thereby making it truly vertical and, in the event of there being no adjusting tools available, the instrument may be used at this stage. The Fig.
Misaligned plate bubble. (a) When levelled over two footscrews, (b) when turned through 180°
Collimation in azimuth
bubble will still be off centre by an amount proportional to e, and should now be centralized by raising or lowering one end of the bubble using its capstan adjusting screws.
(2) Collimation in azimuth:
The purpose of this test is to ensure that the line of sight is perpendicular to the transit axis. Test: The instrument is set up, and levelled, and the telescope directed to bisect a fine mark at A, situated at instrument height about 50 m away (Figure ). If the line of sight is perpendicular to the transit axis, then when the telescope is rotated vertically through 180◦, it will intersect at A1. However, assume that the line of sight makes an angle of (90◦ − e) with the transit axis, as shown dotted in the face left (FL) and face right (FR) positions. Then in the FL position the instrument would establish a fine mark at AL. Change face, re-bisect point A, transit the telescope and establish a fine mark at AR. From the sketch it is obvious that distance ALAR represents four times the error in the instrument (4e.
3) Spire test (transit axis test):
The spire test ensures that the transit axis is perpendicular to the vertical axis of the instrument. Test: The instrument is set up and carefully levelled approximately 50 m from a well-defined point of high elevation, preferably greater than 30◦ (Figure 5.24). A well-defined point A, such as a church spire, is bisected and the telescope then lowered to its horizontal position and the vertical cross-hair is used to mark a point on a peg or a wall. If the transit axis is in adjustment the point will appear at A1 directly below A. If, however, it is in error by the amount e (transit axis shown dotted in FL and FR positions), the mark will be made at AL. The instrument is now changed to FR, point A bisected again and the telescope lowered to the horizontal, to fix point AR. The distance ALAR is twice the error in the instrument (2e).