Surveying Questions and Answers - Balancing Backsights and Foresights

1.If the observed back sight and fore sight are x1 and x2. The correction back sight on A will be equal to x1-y1, where y1= D1 tan i°. The correct fore sight on B will be equal to x2-y2 where, y2 = D2 tan i°. Then what is the correction difference in level between A and B, if D1 = D2?
a) x1 – x2
b) x2 + x1
c) x1 – x2 + (D2 tan i° – D1 tan i°)
d) x2 – x1 + (D1tan i° + D2 tan i°)

Answer: a
Explanation: The correct difference in level between A and B is (x1 – x2) – (x2 – y2) = (x1 – x2) + (y2 – y1) = x1 – x2 + (D2 tan i° – D1 tan i°) = x1 -‘x2.

2. If the observed back sight and fore sight are 20 m and 18 m. The correction back sight on A will be equal to 16 m, The correct fore sight on B will be equal to 14 m where then what is the correction difference in level between A and B?
a) 4 m
b) 3 m
c) 2 m
d) 6 m

Answer: c
Explanation: The correct difference in level between A and B is (20 – 4) – (18- 4) = 20 – 18 = 2 m.

3.If the staff reading at point A = ha and at a point B = hb. The correct staff reading should have been Ha and Hb, then the correction difference in elevation between A and B is given by _____
a) ha – hb
b) ha + hb
c) Ha – Hb
d) Ha + Hb

Answer: c
Explanation: The correction difference in elevation between any two points is given by taking a difference of correct staff reading. Therefore, here Ha – Hb.

4. If the staff reading at point A = ha and at a point B = hb. The correct staff reading should have been Ha and Hb, where Ha = ha – ha’ and Hb = hb – hb’ then the correction difference in elevation between A and B is given by ____________
a) ha – hb – ha’ + hb’
b) ha + hb + ha’ + hb’
c) Ha – Hb + ha’ – hb’
d) Ha + Hb

Answer: a
Explanation: The correction difference in elevation between any two points is given by taking a difference of correct staff reading. Therefore, here Ha – Hb.
Ha – Hb = (ha – ha’) – (hb – hb’) = ha – hb – ha’ + hb’.

5. If the back sight and fore sight distances are balanced, the elevation between two points is equal to the difference between the rod readings taken to the two points and correction for curvature and refraction is necessary.
a) True
b) False

Answer: b
Explanation: If the back sight and fore sight distances are balanced, the elevation between two points is equal to the difference between the rod readings taken to the two points. No correction for curvature and refraction is necessary.

6. Turning point is also called _____
a) intermediate point
b) level point
c) change point
d) end point

Answer: c
Explanation: Turning point is a point on which both minus sight and plus sight are taken on a line of direct levels. It is also called a change point.

7. Horizontal line departs from a level surface because of _____________
a) Refraction
b) Radius of earth
c) Curvature of earth
d) Parallelism

Answer: c
Explanation: From the definition of level surface and a horizontal line it is evident that a horizontal line departs from a level surface because of the curvature of the earth.

8.In the long sights, the horizontal line of sight doesn’t remain straight but it slightly bends downwards having concavity towards earth due to ____________
a) Refraction
b) Radius of earth
c) Curvature of earth
d) Parallelism

Answer: a
Explanation: Due to refraction, in the long sights, the horizontal line of sight doesn’t remain straight but it slightly bends downwards having concavity towards earth.

9. Find the correction for curvature for a distance 1200 m?
a) 0.113 m
b) 0.131 m
c) 0.133 m
d) 0. 313 m

Answer: a
Explanation: Correction for curvature is 0.07849 d2. Therefore, here 0.07849*(1.2)2 = 0.113 m.

10. Find correction for refraction for a distance of 1200 m?
a) 0.0106 m
b) 0.0160 m
c) 0.0016 m
d) 0.0116 m

Answer: b
Explanation: Correction for refraction is correction for curvature/7.
Therefore, here (0.07849 * (1.2)2)/7 = 0.016 m.