Surveying-I,UNIT-III permanent adjestment

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UNIT-III Permanent Adjustment of Level:

UNIT-III Permanent Adjustment of Level Presented by C.Makendran M.E.,(Ph.D)

Introduction:

Introduction An instrument from manufacturer is generally available in perfect adjustments. However, the instrument gets out of adjustments due to worn out and loose fittings or due to mishandling. So it is required to check the instruments occasionally especially before any precise survey work to ensure that the instrument is in perfect adjustment.

Fundamental Lines of a Level :

Fundamental Lines of a Level There are three fundamental lines in a level instrument ( Figure 12.1 ). These are Vertical axis Axis of the level tube Line of sight

Fundamental Lines of a Level :

Fundamental Lines of a Level

Slide 5:

Relations among Fundamental Lines ( Figure 12.2 ) In a properly adjusted dumpy level, desired relations among fundamental lines are Axis of the level tube is perpendicular to the Vertical axis Horizontal cross hair should lie in a plane perpendicular to the Vertical axis, so that it will lie in a Horizontal plane when the instrument is properly leveled. The Line of sight is parallel to the axis of the level tube. Also, the optical axis, the axis of the objective lens and the line of sight should coincide.

Slide 7:

Permanent Adjustment of Level The permanent adjustment of a level is tested by finding the relative position of fundamental lines.

Slide 8:

If any fundamental relation is found to be disturbed in a dumpy level, the cross-hairs and level tube are adjusted so that the fundamental relations get satisfied. The reference line for the adjustments in dumpy level is the vertical line which remain fixed in direction, as it depends upon the direction of gravity.

Axis of the Level Tube is Perpendicular to the Vertical axis :

Axis of the Level Tube is Perpendicular to the Vertical axis Test After setting and leveling the level, turn the telescope through 180 ° about its vertical axis. If the bubble remains central, the axis of the level tube is perpendicular to the Vertical axis. Otherwise, a displacement of the bubble from the central position indicates that the tube is not in adjustment. The amount of displacement is double the amount of error, by the principle of reversion ( Figure 12.3 ).

Adjustment:

Adjustment Step 1: With the help of capstan screw, one end of the level tube is raised or lowered, as needed, so that the bubble is halfway back to the centre position. Step 2: With the help of leveling screws, the other half of the displacement is moved further to bring the bubble at centre. The steps are repeated until the adjustment is perfected.

Horizontal Cross Hair Should Lie in a Plane Perpendicular to the Vertical axis :

Horizontal Cross Hair Should Lie in a Plane Perpendicular to the Vertical axis Test A well-defined point is focused along the horizontal cross hair on one side of the field of view. The instrument (telescope) is then rotated about its vertical axis. If the point appears to travel along the horizontal cross-hair, the instrument is in adjustment i.e., the horizontal cross-hair lies in a plane perpendicular to the Vertical axis. Otherwise, there is a need for adjustment.

Adjustment :

Adjustment Let us rotate the instrument in such a way that the well defined point occupy a position on the opposite side of the field of view, say X' ( Figure 12.4 ). The cross hair ring is then rotated by loosening two adjacent capstan screws. Repeat the process until the point travels along the horizontal cross hair.

The Line of Sight is Parallel to the axis of the Bubble Tube:

The Line of Sight is Parallel to the axis of the Bubble Tube Test Two pegs are set at some distance (of about 60 to 90 m) on a fairly level ground. A dumpy level is set up on a point which is equidistant from the pegs and preferably, in a line with the pegs. Staff readings are taken at the pegs, say the readings are a and b respectively. Then, the true difference in elevation between the points is h = ( ab ).

The Line of Sight is Parallel to the axis of the Bubble Tube:

The Line of Sight is Parallel to the axis of the Bubble Tube Now, the instrument is set on the line joining the pegs near one of the pegs but opposite to the other peg, as shown in Figure 12.5 . Let D 1 and D 2 are the distances of the near and far peg from the instrument position. Staff readings are again taken at the pegs, say the readings are c and d respectively.

The Line of Sight is Parallel to the axis of the Bubble Tube:

The Line of Sight is Parallel to the axis of the Bubble Tube Then, the apparent difference in elevation between the points is h' = ( cd ). Now, if h' is found to be equal to h, the line of sight of the level is parallel to the axis of the bubble tube. Otherwise, an adjustment of the bubble tube is required.

The Line of Sight is Parallel to the axis of the Bubble Tube:

The Line of Sight is Parallel to the axis of the Bubble Tube

Adjustment :

Adjustment Step 1: The amount of error (e) associated with the observation is determined from e = hh ' = ( ab ) – ( cd ) Step 2: The error e occurs in a distance of D = D 2 - D 1 . Assuming the error e is positive and the line of sight is inclined upward, the error in distance D 2 Step 3: Calculate the correct staff reading at the distant peg as

Adjustment :

Adjustment Step 4: The capstan screws at the top and bottom of the diaphragm ring is then loosened and the ring is moved vertically so that the line of sight intersect the distant staff at d'. Step 5: To check the adjustment, read the staff reading on the near peg and it should read If the near staff reading is not c', then repeat Step 4 and till Step 5 is satisfied. This method is known as two peg method. In this method, due account need to be taken about sign throughout the test.

Permanent Adjustment of Automatic Level :

Permanent Adjustment of Automatic Level In case of automatic level, prior to leveling, the circular bubble and the activity of the compensator are required to be checked. In this case, after setting the instrument, the circular bubble is brought to the centre of the engraved circle. Then, the instrument is rotated through 180° or to a direction in which the displacement of the bubble is maximum. If the displacement is less than half the diameter of the bubble, the instrument is in adjustment. Otherwise, an adjustment is necessary

Permanent Adjustment of Automatic Level :

Permanent Adjustment of Automatic Level Adjustment is performed in two steps by reducing the horizontal and vertical components of the displacement and bringing the bubble to the centre of the engraved circle. First remove the half of horizontal component of displacement using the appropriate level screws.and then remove the remaining half by manipulating the adjusting screws.

Permanent Adjustment of Automatic Level :

Permanent Adjustment of Automatic Level Next, one-half the vertical component of displacement ( Figure 12.6 ) is then removed by the level screws and the remainder is compensated using the adjusting screws to bring the bubble to the center of the engraved circle. Turn 180° and check the bubble. Repeat the entire procedure until the bubble remains centered in all positions.

Permanent Adjustment of Automatic Level :

Permanent Adjustment of Automatic Level

Permanent Adjustment of Automatic Level :

Permanent Adjustment of Automatic Level To check the compensator, sight a target about 30 m away. Center the bull's-eye bubble. Next, tap one tripod leg with a hand. The image of the target will appear to swing in the field of view but the target will return to its original position. Next, turn a level screw, causing the line of sight to slope slightly.

Permanent Adjustment of Automatic Level :

Permanent Adjustment of Automatic Level Once again the target will appear to swing but will return to the original Position. This indicates that the instrument is in adjustment. Otherwise, an adjustment is necessary. For automatic level also, the third fundamental relation is tested by two peg test but for permanent adjustment manual of the instrument is required to be referred.

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