Surveying Questions and Answers for Campus interviews on “Levelling – Balancing Backsights and Foresights”.
1. By balancing back sight and fore sight error due to curvature can be eliminated.
a) True
b) False
Answer: a
Clarification: When the difference in elevation between any two points is determined from a single set up back sighting on one point and fore sighting on the other, error due to curvature can be eliminated. Error due to refraction also be eliminated.
2. By balancing back sight and fore sight error due to non parallelism of the line of collimation can be eliminated.
a) True
b) False
Answer: a
Clarification: When the difference in elevation between any two points is determined from a single set up back sighting on one point and fore sighting on the other, error due to non parallelism of the line of collimation can be eliminated. Error due to refraction also is eliminated.
3. By which of the following, the difference in elevation between two points can be calculated by taking a difference between the two readings and no correction for the inclination of the line of sight is necessary?
a) Levelling
b) Centering
c) Contouring
d) Balancing
Answer: d
Clarification: If the back sight and foresight distances are balanced, the difference in elevation between two points can be directly calculated by taking a difference of two readings and no correction for the inclination of the line of sight is necessary.
4. 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.
a) x1 – x2
b) x2 – x1
c) x1 – x2 + (D2 tan i° – D1 tan i°)
d) x2 – x1 + (D1tan i° + D2 tan i°)
Answer: c
Clarification: 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°).
5. 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
Clarification: 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.
6. 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
Clarification: The correct difference in level between A and B is (20 – 4) – (18- 4) = 20 – 18 = 2 m.
7. 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