Surveying (Part-2) (General Engineering) (SSC JE)

Parallax– Parallax is the observed displacement of an object caused by the change of the observer's point of view. Removal of parallax may be achieved by focusing the objective and eye piece.

Invar–Invar tapes are made of an alloy which consists of 36% of nickel and 64% of steel. The coefficient of thermal expansion is very low compared to other tapes. The invarof steel used for precision of leveling staff is quantity.

Errors of instruments– It can be determined by a process known as calibration. Calibration is the companion of measurement values delivered by a device under test with those of a calibration standard of known accuracy.
Error = instrument reading – true reading

Instrument error are classified into three categories–
Gross errors– Arise due to human mistakes
Random errors– Cause of such errors are clearly unknown.
Systematic errors– Affect all the reading in a particular fashion.
Zero error is an example for systematic error.

Prismatic compass– Prismatic compass is the most convenient and portable form of magnetic compass which can either be used as a hand instrument or can be fitted on a tripod.

It will orient itself in the magnetic meridian and therefore, the N and S ends of the ring will be in this direction.

• The 0º or 360º reading is, therefore, engraved on the south end of the ring. The reading in "clockwise" direction from 0º at south end to 90º at west end, 180º at north end and 270º at east end.

Optical square–
1. It is more accurate than the cross staff and is used for setting out accurately the long offsets.
2. It is a small compact hand instruments based upon the "principle of reflection".
3. Horizontal glass and index glass are two mirror placed at an angle of 45º to each other.
4. In case of an optical square, the angle between the first incident ray and the last reflected ray is 90º.

Check line–A line joining the apex of a triangle to some fixed point on the opposite side or a line joining some fixed points on any two side of triangle.
• A check line is measured to check the accuracy of framework.

Base line–A line joining some fixed point on the main survey line.
• Check the accuracy of the framework.
• The surveyor to locate the interior details which are for away from the main chain lines.

Axis of leveled tube– The axis of the level tube or the bubble line is a straight line tangential to the longitudinal curve of the level tube at its centre.
• The axis of the level tube is horizontal when the bubble is central.

Line of collimation– It is the imaginary straight line which joins the optical center of the object glass with the point of intersection of cross hairs of the diaphragm.
• If line of sight is horizontal then it is called line of collimation.
Line of sight–It is the imaginary line joining the intersection of the cross hairs of the diaphragm to the optical center of the object glass and its continuation.

Repetition method
• The repetition method is used to measure a horizontal angle very accurately. • The precision attained is to a much finer degree than the least count of a vernier.
• In this method, the horizontal angle is measured several time and the value is added mechanically.
• The value horizontal angle is equal to the accumulated value divided by the number of repetition done.
Note–In this method certain error in the theodolite are also eliminated.

Level line–A level ine is a line lying in a level surface. It is, therefore, normal to the plumb line at all points.

Horizontal line–It is straight line tangential to the level line at a point. It is also perpendicular to the plumb line.

The diaphragm of a surveying telescope is typically held inside the telescope at the end nearest the eyepiece.
• The diaphragm is a small, adjustable aperture that is used to control the amount of light that enters the telescope.