Horizontality of the Line of Sight

TM 5-6675-329-13 & P

(1)

In flat terrain a test bay, between 45 and 60 m long (about 150 to 200 ft.), is selected and divided into

three equal sections of length d. A staff is set up on an iron ground plate or peg at each of the intermediate points, B and

C. (If only one staff is used it must be moved as required from peg B and peg C.) The instrument is set up, in turn, over

the terminal points A and D.

With the instrument at A after centering the circular bubble and checking the compensator with the

pushbutton (11) readings a'1 and a'2 are taken to the staffs held at B and C. With the instrument at D, readings a'3 (to C)

and a'4 (to B) are then taken. If the line of sight is absolutely horizontal, these readings will be the correct readings a1, a2,

a3, and a4 respectively, and the following relationship will be valid, as can be seen from the figure:

a4 - a1 = a3 - a2

If this is not so, however, the line of sight is inclined to the horizontal plane by the small angle, δ. If an

imaginary line, parallel to a'2 a'1 is projected through a'3, it will cut the staff at B in the correct position a4 , thus giving the

required value for a true, horizontal line of sight from D. This is seen quite plainly in the figure.

a4 - a1 = a' - a'

giving a4 = a'1 - a'2 + a'3

If the actual reading a'4 differs from the computed, correct value a4 by more than 2 mm in 30 m (or 0.005 ft. in 100 ft.),

the whole procedure must be repeated. If the difference is confirmed, the line of sight must be adjusted as described in

step (2) below.

The tolerance of 2 mm is 30 m (0.005 ft. in 100 ft.) is a practical value for routine leveling with the instrument.

Figure 4-2. Testing the Horizontally of the Line of Sight

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