For equipment to give the best possible results it should be frequently tested and, if necessary, adjusted. Surveying equipment receives continuous and often brutal use on construction sites. In all such cases a calibration base should be established to permit weekly checks on the equipment.
The tilting level requires adjustment for collimation error only. Collimation error occurs if the line of sight is not truly horizontal when the tubular bubble is centred, i.e. the line of sight is inclined up or down from the horizontal.Acheck known as the ‘Two-Peg Test’ is used, the procedure being as follows (Figure ):
(a) Set up the instrument midway between two pegs A and B set, say, 20mapart and note the staff readings, a1 and b1, equal to, say, 1.500 m and 0.500 m respectively.
Let us assume that the line of sight is inclined up by an angle of α; as the lengths of the sights are equal (10 m), the error in each staff reading will be equal and so cancel out, resulting in a ‘true’ difference in level between A and B.
HTRUE = (a1 − b1) = (1.500 − 0.500) = 1.000 m
Thus we know that A is truly lower than B by 1.000 m. We do not at this stage know that collimation error is present.
(b) Move the instrument to C, which is 10 m from B and in the line AB and observe the staff readings a2 and b2 equal to, say, 3.500 m and 2.000 m respectively. Then
H = (a2 − b2) = (3.500 − 2.000) = 1.500 m
Now as 1.500 = the ‘true’ value of 1.000, it must be ‘false’.
HFALSE = 1.500 m
and it is obvious that the instrument possesses a collimation error the amount and direction of which is as yet still unknown, but which has been revealed by the use of unequal sight lengths CB (10 m) and CA (30 m). Had the two values for H been equal, then there would be no collimation error present in the instrument.
(c) Imagine a horizontal line from reading b2 (2.000 m) cutting the staff at A at reading a3. Because A is truly 1.000 m below B, the reading at a3 must be 2.000 1.000 = 3.000 m. However, the actual reading was 3.500 m, and therefore the line of sight of the instrument was too high by 0.500 m in 20 m (the distance between the two pegs). This is the amount and direction of collimation error (d) Without moving the instrument from C, the line of sight must be adjusted down until it is horizontal.
To do this one must compute the reading (a4) on staff A that a horizontal sight from C, distance 30 m away, would give.
By simple proportion, as the error in 20 m is 0.500, the error in 30 m = (0.500×30)/20 = 0.750 m.
Therefore the required reading at a4 is 3.500 − 0.750 = 2.750 m.
(e) (i) Using the ‘tilting screw’, tilt the telescope until it reads 2.750 m on the staff. (ii) This movement will cause the tubular bubble to go off centre. Re-centre it by means of its adjusting screws, which will permit the raising or lowering of one end of the bubble.
The whole operation may be repeated if thought necessary.
The above process has been dealt with in great detail, as collimation error is one of the main sources of error in the levelling process. The diagrams and much of the above detail can be dispensed with if the following is noted:
(1) (HFALSE − HTRUE) = the amount of collimation error.
(2) If HFALSE > HTRUE then the line of sight is inclined up and vice versa.
An example follows which illustrates this approach.