Levelling – Field Work

The survey work is mainly divided into two: (1) field work and (2) office work.

In the field, necessary adjustments are done and the results are recorded in a systematic manner as explained below.

Adjustments of a level

A level needs two type of adjustments, i.e.,

1. Temporary adjustments
2. Permanent adjustments

Temporary adjustments

The adjustments which are made for every setting of a levelling instrument are called temporary adjustments. These include the following:

1. Setting up the level
2. Levelling up
3. Elimination of parallax

Setting up the level

This operation includes fixing the instrument on the tripod and also levelling the instrument approximately by leg adjustments. Setting may be accomplished according to the following steps.

Step 1	The tripod is unfolded and set up on the ground by inserting the metal pointed leg bottoms into the ground so that it stands erect and firm to hold the level at a height convenient to the observer looking through the telescope.
Step 2	The box containing the level is opened and the position of the level as it suits in the box is noted. If required the edges of the box are marked so that the level may be replaced in the box correctly.
Step 3	The level is set on the stand by screwing it on or clamping it in the slot of the tribrach.
Step 4	One hand is placed lightly on the telescope and the other hand on one of the legs, pressed against the thigh, to avoid jerks and snaps, while the leg is given lateral to and fro motion to bring the bubble in the tube on the tribrach in the centre of its run, or to bring the bubble to the centre of a circular level on the tribrach, if such a level is provided on it.
Step 5	The movement of the legs is now locked with the help of any suitable mechanism provided with the tripod.

Levelling up

The vertical axis of the instrument is rendered truly vertical through the levelling up operation which may be accomplished in accordance with the following steps. In Figure 27.11 (a) and (b) three-foot screw arrangements of a dumpy level and in figures (c) and (d) four-foot screw arrangements of a dumpy level are shown.

Step 1	The telescope axis is placed parallel to the line joining one pair of screws as in Figure 27.11(a) and in Figure 27.11(c).
Step 2	Both the screws 1 and 2 are rotated simultaneously by moving them equally inwards or equally outwards till the bubble of the spirit level on the telescope is brought to the centre of its run.
Step 3	The telescope axis is now placed perpendicular to its previous position, i.e., perpendicular to 1-2 line.
Step 4	The third screw 3 in Figure 27.11(b) (in the case of a three-screw system) is rotated to bring the bubble of the spirit level on the telescope to the centre of its run.

In the case of four-screw arrangements, the screws 3 and 4 are to be moved equally inward or equally outward till the bubble of the telescope spirit level is brought to the centre of its run (see Figure 27.11(d)).

Step 1 through 4 have to be repeated several times before the bubble tube of the spirit level on the telescope remains central for all positions of the telescope when the vertical axis of the instrument becomes truly vertical. In reality, however, if the bubble remains central over the angle of rotation, of the telescope, required for reading the levelling staff, the purpose is served.

Elimination of parallax

An apparent change in the position of the object caused by the change in position of the observer’s eye is known as parallax. In a telescope, parallax is caused when the image formed by the objective is not situated in the plane of the cross hairs. Unless the parallax is removed, accurate bisection and sighting of objects become difficult. Elimination of parallax is done by focusing the eyepiece for distinct vision of cross hairs and focusing the objective to bring the image of the object in the plane of the cross hairs as explained below.

 

 

Figure 27.11 Levelling the instrument

 

• Focusing the eyepiece: To focus the eyepiece for a distinct vision of cross hairs, either hold a white paper in front of the objective or sight the telescope towards the sky. Move the eyepiece in or out till the cross hairs are seen sharp and distinct.
• Focusing the objective: After the cross hairs have been properly focused, direct the telescope on a well-defined distinct object and intersect it with a vertical wire. Focus the objective till a sharp image is seen. Moving the eye slowly to one side may check removal of the parallax. If the object still appears intersected, there is no parallax. If on moving the eye laterally, the image of the object appears to move in the same direction as the eye and the observer’s eye and the image of the object are on the opposite sides of the vertical wire, the image of the object and the eye are brought nearer to eliminate the parallax. This parallax is called far-parallax. If, on the other hand, the image appears to move in a reverse direction to the movement of the eye and the observer’s eye and the image of the object are on the same side of the vertical wire, then the parallax is called near-parallax. It may be removed by increasing the distance between the image and the eye.

Classification of levelling

Levelling may be classified into two categories:

1. Simple levelling
2. Differential levelling

Simple levelling

When two points whose level difference is to be found out are situated in such a way that both of them are visible from a single position of level, this method is adopted.

 

 

Figure 27.12 A simple levelling work

 

Suppose A and B are two such points and the level is set up midway between A and B. After the instrument is correctly levelled, the telescope is directed towards the staff held vertically on A and focused. The reading at which the horizontal hair of the diaphragm appears to cut the staff is then taken carefully. Make sure that the bubble is central while the staff is being read. Then the staff is held vertically on B and the telescope of the level is directed on to the staff held at B, is focused and the reading at B is noted. Let the respective reading on A and B be 2.200 and 0.565. The difference between these two readings gives the difference in level between A and B, which is equal to 2.200 – 0.565 = 1.635 m (Figure 27.12).

Assume the reduced level of A as 100.000 m.

 

Height of instrument at station C	= Reduced level atof A + staff reading at A
	= 100.00 + 2.200 = 102.200 m

Reduced Level of B = 102.200 – 0.565 = 101.635 m

Differential levelling

The method of levelling for determining the difference in elevation of two points either too far apart or obstructed by an intervening ground is known as differential levelling. In this method, the level is set at a number of points and the difference in elevation of successive points is determined as in the case of simple levelling. This levelling process is also known as fly levelling, compound levelling or continuous levelling.

Hence, this method is adopted when the points are too far apart, if the difference in elevation between them is too great or if there are obstacles between them.

In this case, the level is set up at different positions (points) for the execution of the levelling operations as in Figure 27.13 below. Consider two points A and E as in Figure 27.13; it is required to find the level difference between these two points. Set up the level at a convenient point, let it be at ‘a’. The work is started by taking back sight to a bench mark or a known point and then fore sight is taken to a point to fix its level at A. Take a staff reading at A, let it be ‘a1’. Select a firm point b, so that the distance from C to ‘b’ is approximately equal to the distance from A to ‘a’. Hold the staff at B and take the staff reading. Let it be ‘b1’. This forms first stage in the levelling series. Keeping the staff at B, shift the instrument to ‘b’. Take a back sight at B, let it be ‘b2’. With the levelling instrument at ‘b’ shift the levelling staff to a third position at ‘C’ and the work is continued till the point E is read.

 

 

Figure 27.13 Execution of a direct levelling operation

 

The staff reading ‘a1’ taken at A from the instrument station ‘a’ is the back sight and the staff reading ‘b1’ taken at B is the fore sight. The staff reading ‘b2’ taken at B from the instrument station ‘b’ is the back sight and the staff reading ‘c1’ taken at C is the fore sight and so on.

Hence, the level difference between A and B = a1 – b1.

The level difference between B and C = b2 – c1 and so on.

The difference of level between A and E is equal to the algebraic sum of these differences or equals to the difference between the sum of the back sights and the sum of the fore sights.