Surveying-1 unit III

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UNIT-III LEVELLING (Surveying-I):

UNIT-III LEVELLING (Surveying-I) Prepared by C. Makendran M.E.,(Ph.D)

INSTRUMENT:

INSTRUMENT The instrument primarily used for leveling is the (engineer's) level in association with a graduated rod known as leveling rod or leveling staff.

Slide 4:

A schematic diagram of an engineer's level is shown in Figure 11.1 . An engineer's level primarily consists of a telescope mounted upon a level bar which is rigidly fastened to the spindle. Inside the tube of the telescope, there are objective and eye piece lens at the either end of the tube. A diaphragm fitted with cross hairs is present near the eye piece end .

Slide 5:

A focussing screw is attached with the telescope. A level tube housing a sensitive plate bubble is attached to the telescope (or to the level bar) and parallel to it. The spindle fits into a cone-shaped bearing of the leveling head. The leveling head consists of tribrach and trivet with three foot screws known as leveling screws in between. The trivet is attached to a tripod stand.

Functions of Salient Parts :

Functions of Salient Parts Telescope : used to sight a staff placed at desired station and to read staff reading distinctly. Diaphragm : holds the cross hairs (fitted with it). Eye piece : magnifies the image formed in the plane of the diaphragm and thus to read staff during leveling. Level Tube : used to make the axis of the telescope horizontal and thus the line of sight. Leveling screws : to adjust instrument (level) so that the line of sight is horizontal for any orientation of the telescope. Tripod stand : to fix the instrument (level) at a convenient height of an observer.

Dumpy Level :

Dumpy Level A dumpy level is most suitable when from one setting of the instrument, elevations of several points are to be determined. ( Figure 11.2 )

Distinctive Features of an Dumpy Level :

Distinctive Features of an Dumpy Level The optical axis of the telescope of a dumpy is placed perpendicular to the axis of the centre spindle. The axis of its level tube is permanently placed so that it lies in the same vertical plane as the optical axis

A IOP level is most suitable when only few readings are to be taken from one setting of the instrument. :

A IOP level is most suitable when only few readings are to be taken from one setting of the instrument .

Distinctive Features of an IOP Level :

Distinctive Features of an IOP Level The telescope is mounted on a transverse fulcrum at the vertical axis fitted with a micrometer screw at the eye-piece end of the telescope. Instrument is leveled using circular spirit level. The sensitive plate-bubble is to be leveled using micrometer screw, at the time of taking measurement. Thus, the line of sight is made horizontal quickly, even though the instrument as a whole may not be exactly level.

Digital level:

Digital level There are fundamentally two types of automatic levels. First, the optical one whose distinguishing feature is self-leveling i.e., the instruments gets approximately leveled by means of a circular spirit level and then it maintains a horizontal line of sight of its own. Second, the digital levels whose distinguishing features are automatic leveling, reading and recording

Slide 15:

The chief features of a digital level are: A CCD (Charged coupled device) at the plane of diaphragm. It captures an image of the rod and processes it resulting in a rod reading and a distance to the rod. A data collector which keeps the level notes, performs checks and keeps a record of every rod reading and elevation automatically. A bar-coded rod having a scale represented through a series of bars of different widths. Bars are spaced constantly or variably. The spacing and width of the bars denote the code.

Slide 16:

Advantages of digital levels include the speed of leveling, the virtual elimination of rod reading and calculation errors and the accuracy in reading rod. Limitation of digital level lies in its range. Beyond a certain limit it is to be used in “manual mode”.

Leveling Staff:

Leveling Staff It is a self-reading graduated wooden rod having rectangular cross section. The lower end of the rod is shod with metal to protect it from wear and usually point of zero measurement from which the graduations are numbered. Staff are either solid (having single piece of 3 meter height) ( Figure 11.5 ) or folding staff (of 4 meter height into two or three pieces) ( Figure 11.6 ). The least count of a leveling staff is 5 mm.

Temporary Adjustment of Level:

Temporary Adjustment of Level At each set up of a level instrument, temporary adjustment is required to be carried out prior to any staff observation . It involves some well defined operations which are required to be carried out in proper sequence.

Slide 19:

The temporary adjustment of a dumpy level consists of Setting , Leveling and Focusing . During Setting , the tripod stand is set up at a convenient height having its head horizontal (through eye estimation).

Slide 20:

The instrument is then fixed on the head by rotating the lower part of the instrument with right hand and holding firmly the upper part with left hand. Before fixing, the leveling screws are required to be brought in between the tribrach and trivet. The bull's eye bubble (circular bubble), if present, is then brought to the centre by adjusting the tripod legs.

Leveling of the instrument is done to make the vertical axis of the instrument truly vertical. It is achieved by carrying out the following steps: :

Leveling of the instrument is done to make the vertical axis of the instrument truly vertical. It is achieved by carrying out the following steps: Step 1: The level tube is brought parallel to any two of the foot screws, by rotating the upper part of the instrument. Step 2: The bubble is brought to the centre of the level tube by rotating both the foot screws either inward or outward. (The bubble moves in the same direction as the left thumb.) Step 3: The level tube is then brought over the third foot screw again by rotating the upper part of the instrument. Step 4: The bubble is then again brought to the centre of the level tube by rotating the third foot screw either inward or outward.

Slide 22:

Step 5: Repeat Step 1 by rotating the upper part of the instrument in the same quadrant of the circle and then Step 2. Step 6: Repeat Step 3 by rotating the upper part of the instrument in the same quadrant of the circle and then Step 4. Step 7: Repeat Steps 5 and 6, till the bubble remains central in both the positions. Step 8: By rotating the upper part of the instrument through 180 ° , the level tube is brought parallel to first two foot screws in reverse order. The bubble will remain in the centre if the instrument is in permanent adjustment.

Slide 23:

Focusing is required to be done in order to form image through objective lens at the plane of the diaphragm and to view the clear image of the object through eye-piece. This is being carried out by removing parallax by proper focusing of objective and eye-piece. For focusing the eye-piece, the telescope is first pointed towards the sky. Then the ring of eye-piece is turned either in or out until the cross-hairs are seen sharp and distinct.

Slide 24:

Focusing of eye-piece depends on the vision of observer and thus required whenever there is a change in observer. For focusing the objective, the telescope is first pointed towards the object. Then, the focusing screw is turned until the image of the object appears clear and sharp and there is no relative movement between the image and the cross-hairs. This is required to be done before taking any observation.

Temporary Adjustment of an IOP Level:

Temporary Adjustment of an IOP Level Temporary adjustment of a tilting level requires the same operations as in case of a dumpy level except the operations involved in leveling. During leveling, first the IOP instrument is leveled roughly with the leveling screws till the circular bubble is in the centre. Then, the bubble of the level tube is brought to the centre by using the tilting screw. In case of IOP level, the bubble is required to be leveled using tilting screw before each reading is taken.

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