Question

Which of the following statements is NOT true for precise levelling when compared to ordinary levelling?
1. High grade levels and stadia rods are used in precise levelling.
2. Two rodmen are employed and backsight and foresight are taken in quick succession.
3. Length of sight is limited to 100 m in length.
4. Rod readings are taken against the two horizontal hairs of the diaphragm.

Answer 1

Correct Answer - Option 4 : Rod readings are taken against the two horizontal hairs of the diaphragm.

Concepts:

Precise Leveling is used for establishing bench marks with high precision at widely distant points.

• It requires the use of highly modern instruments and greatest care in the field.  
• A high grade level equipped with tilting screw, stadia wires and coincidence level etc. and an invar precision leveling staff are commonly required.
• The parallax should be entirely eliminated by correct focusing.
•  The staff should be exactly vertical.  
•  The bubble should be exactly in the centre of its run at the time of taking readings.
•  Lengths of sights are limiting to about 100 m.
• The back sight and fore sight distances should be exactly equal. Stadia readings may be taken for this purpose.
• To avoid error due to settlement of tripod and staff, the back sights and the following fore sights should be taken in quick successions and the order of taking readings is interchanged at alternate set up i.e. at first setting, the back sight is observed first and then the fore sight while at the 2nd setting, the foresight is taken first and then the back sight and so on.

Rod readings are taken against the two horizontal hairs of the diaphragm. This is done in Tachometry Survey in which horizontal and vertical distances are taken.