Which levelling instrument have the high presion, the mean elevation error on invar staff graduated to 5mm division varies between +0.5 - 0.8 per km of forward and back ward levelling.

The stations choosed by main survey lines called . . . . . . . .

A. Temporary stations

B. Main stations

C. Subsidary station

D. None

The drawing of an irregular area is covered by 42.2 squares of 200 mm side. If the side of the drawing is 10mm = 1m, the actual area of field . . . . . . . .

A. 16880m²

B. 8440m²

C. 1688m²

D. 844m²

The 90° corners of a building are set out on the ground by, easily . . . . . . . .

A. Cross staff

B. Mason's square

C. Theodolite

D. Optical square

The arithmetical check for levelling is . . . . . . . .

A. $$\sum {BS} + \sum {FS = {\text{First RL + Last RL}}} $$

B. $$\sum {FS} - \sum {BS} = {\text{Last RL + First RL}}$$

C. $$\sum {BS} - \sum {FS} = {\text{Last RL}} - {\text{ First RL}}$$

D. All