Civil Engineering - Surveying - Discussion
Discussion Forum : Surveying - Section 3 (Q.No. 34)
34.
For orientation of a plane table with three points A, B and C, Bessel's drill is
Discussion:
2 comments Page 1 of 1.
Sabin Chaudhary said:
2 years ago
Anyone, please explain this.
Xaher said:
1 week ago
The correct option is Align b through a and draw a ray towards c, align a through b and draw a ray towards c.
Finally align c through the point of intersection of the previously drawn rays.
Explanation.
Bessel's drill for the three-point problem in plane table surveying involves a specific sequence of operations to orient the table correctly when its position is unknown, using three known points (A, B, C) that are already plotted on the map as a, b, and c.
The method is a trial-and-error process involving drawing rays (resectors) and adjusting the table's orientation until the rays from all three points intersect at a single point, which represents the current plane table station on the map.
The steps provided in the correct answer describe one specific and correct sequence for this iterative process:
Align the alidade along the line joining the plotted points a and b, and then sight the ground point B through A to orient approximately.
Draw a ray towards ground point C.
Next, align the alidade along the line a and b again, and then sight ground point A through B.
Draw a ray towards ground point C.
The intersection of the two rays drawn toward C gives the approximate position of point P (the plane table station).
Finally, align the alidade through this intersection point and the plotted point c to sight the ground point C, thus ensuring correct orientation.
Why other options are incorrect.
The other options describe different sequences of steps that do not conform to the established procedure of Bessel's method, which is designed to graphically find the unique correct orientation and position of the plane table. The specific order of sighting and drawing rays is crucial for the method to work as intended and converge on the correct solution.
Finally align c through the point of intersection of the previously drawn rays.
Explanation.
Bessel's drill for the three-point problem in plane table surveying involves a specific sequence of operations to orient the table correctly when its position is unknown, using three known points (A, B, C) that are already plotted on the map as a, b, and c.
The method is a trial-and-error process involving drawing rays (resectors) and adjusting the table's orientation until the rays from all three points intersect at a single point, which represents the current plane table station on the map.
The steps provided in the correct answer describe one specific and correct sequence for this iterative process:
Align the alidade along the line joining the plotted points a and b, and then sight the ground point B through A to orient approximately.
Draw a ray towards ground point C.
Next, align the alidade along the line a and b again, and then sight ground point A through B.
Draw a ray towards ground point C.
The intersection of the two rays drawn toward C gives the approximate position of point P (the plane table station).
Finally, align the alidade through this intersection point and the plotted point c to sight the ground point C, thus ensuring correct orientation.
Why other options are incorrect.
The other options describe different sequences of steps that do not conform to the established procedure of Bessel's method, which is designed to graphically find the unique correct orientation and position of the plane table. The specific order of sighting and drawing rays is crucial for the method to work as intended and converge on the correct solution.
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