PERMANENT ADJUSTMENTS

SURVEYING - 1

INTRODUCTION

OBJECTIVE

The object of permanent adjustment of level is to establishthe fixed relationships between itsfundamental lines.

The fundamental axis of leveling instruments are:

1) The axis of bubble tube2) The line of collimation3) The vertical axis4) The axis of telescope

The desired relationships are:

(1) The line of collimation should be parallel to bubble tube axis

(2) The line of collimation and axis of telescope should coincide with one another

(3) The bubble axis should be perpendicular to vertical axis so that the bubble traverses i.e. remains in the central position for all directions of the telescope.

Each adjustments involves2 steps:

(1) A test to determine the error

(2) An adjustment or correction of this error

PRINCIPLE OF REVERSAL This principle states that,“If any error exists in certain partsof the instrument, it is doubled on reversing i.e. revolving through 180the position of that part.”

Another use of this methodis that , even though the instrumentis out of adjustment , accurate results can be obtained by reversing and taking the mean of two observations.

FIRST ADJUSTMENT

TO MAKE THE AXIS OF THE BUBBLE TUBE PERPENDICULARTO VERTICAL AXIS

The object of this adjustments is to ensure that if the instrument isonce levelled up, the bubble remains incentre of its run for all positions of thetelescope.

OBJECT

NECESSITY(1) The adjustment is made only for the convenience of taking reading quickly. (2) Since, it is necessary that the bubble should be central while taking any reading.(3) Much time is wasted if these adjustment is not made, as in that case bubble has to be brought in centre every time for each pointing of telescope.

TEST(a) Set-up the level on firm ground and level it carefully by tripod legsand foot screws.

(b) The bubble will now be central in 2 positions at right angles to each other.one being parallel to pair of foot screw and other over third foot screw.

(c) Bring the telescope over a pair of foot screws or over the third foot screw.

(d) Turn it through 180 in horizontal plane.

(e) If the bubble still remains central, the adjustment is correct.

ADJUSTMENTS(1) If the bubble does not remain in the centre, note down the deviations of the bubble from the centre. say it is ‘2n’ divisions over the bubble half way back i.e. n divisions by raising or by lowering end of bubble tube by means of capsten headed nuts and remaining half with the pair of foot screws beneath the telescope at Its present position.

(2) Turn the telescope through 90. So that it lies over single foot screw below the telescope or parallel to pair of this foot screws.

Bring the bubble in centre of its run by means of these screw pair of foot screws.

(3) Rotate the telescope and see if the bubble remains central for all positions of the telescope.

If not , repeat the whole process until the adjustment is correct.

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