# Surveying and Geomatics Solved Questions | QnA | Part 2 | Basic Surveying | By Akhand Dutta

Bowditch’s Rule: It is also known as the compass rule and is most commonly used in traverse adjustment. It is used to balance the traverse.

## What is the Compass Rule in Surveying?

Ans- (1) Bowditch’s Rule: It is also known as the compass rule and is most commonly used in traverse adjustment.

It is used to balance the traverse when angular and linear measurements are equally precise. It is the rule most commonly used in traverse adjustment.

By this rule, the total error in latitude and that in departure is distributed in proportion to the lengths of the sides.

Correction to latitude or departure of any side

## State any four instrumental errors and four personal errors in the prismatic compass survey.

 Compass

## List the errors occur during prismatic compass survey.

Ans- Four Instrumental Errors:-

1. The errors are caused in sighting and reading if the plane of sight not being vertical.

2. Reading may not be accurate if the magnetic the needle is not perfectly straight.

3. The graduated ring may not be horizontal.

4. Line of sight may not be passing through the centre of the graduated circle.

Four Personal Error:-

1. Inaccurate levelling.

3. Inaccurate centring.

## How to overcome an obstacle in chaining, where vision and chaining both are obstructed?

Ans- Such a problem arises when a building comes across the chain line.

Suppose AB is the chain line. Two points C and D are selected on it one side of the building. Each perpendiculars CC1 and DD1 are erected. The line C1D1 is extended until the building is crossed. On the extended line, two points E1 and F1 are selected. Then perpendicular E1E and F1F are so erected that

E1E = F1F = D1D = C1C

Thus, the points C, D, E and F will lie on the same straight line AB.

The distance D1E1 is measured and is equal to the required distance DE.

## List out any four Accessories of the plane table with their uses.

Ans- (1) Alidade- It is a straight edge ruler having some form of sighting device. One edge of the ruler is bevelled and is graduated. Always this edge is used for drawing a line of sight.

(2) Plumb Bob or Plumbing Fork- A plumbing fork is a U-shaped metal frame with an upper horizontal arm and a lower inclined arm. The plumb bob helps in transferring the ground point to the drawing sheet and vice versa also.

(3) Spirit Level- A flat-based spirit level is used to level the plane table during surveying.

(4) Trough Compass- Trough Compass When the needle ends to coincide with zero-zero, the line of needle ends is parallel to the long edge of the box. Hence, by using it observer gets a marking on either side of a long edge which indicates the north direction of the survey.

## State four Merits and four Demerits of plane table survey.

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Ans- Merits:

(1) Field-notes are not required, and thus the possibility of mistakes in the booking is eliminated.

(2) Measuring of lines and angles is mostly dispensed with since they are obtained graphically.

(3)The surveyor is fully confident about the true representation of the area since he can always compare his work with the actual features on the ground and cannot, therefore, overlook any essential detail.

(4) It is particularly suitable for filling in details in hilly areas and in magnetic areas where chain and compass surveys are not suitable.

Demerits:

(1) It is not useful for large scale surveys and accurate work.

(2) It is not suitable for surveying a densely wooded area.

(3) The absence of field-notes is sometimes inconvenient if the survey has to be replotted to a different scale.

(4) Only day time can be availed of for the field and plotting work whereas, in other methods of surveying, day time can be used for fieldwork and night or even hot and rainy days can be utilised for plotting.

## Discuss the Intersection method of the plane table survey.

Ans- In this method, the plane table is shifted to a known distance in a particular direction marked on the ground and the line of sights are drawn to make the intersection of the radial lines already drawn from the first set up of the instrument.

PROCEDURE:

1) Select two points P and Q such that the points (building corners) to be plotted are visible from their stations.

2) Set the table on P and locate it on the sheet.

3) Pivot on P bisect Q draw a ray.

4) Measure the distance PQ and locate Q on the sheet to a convenient scale.

5) Now PQ is known as the baseline.

6) Pivot ‘P’ bisects the inaccessible objects A and B (building corners) and draw rays.

7) Shift the table to ‘A’ such that P is over Q and do temporary adjustments.

8) Place the alidade along with QP and rotate the table till P is bisected, clamp table.

9) Pivot on Q bisect the objects A and B and draw rays.

10) The instruction of rays drawn from P and Q will give the points A and B.

11) To check the accuracy measured AB and compare with plotted distance AB.

12) The same procedure is applied for other features of the campus. each point is bisected from two stations.

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Ans- 1) The telescopic alidade is used when it is required to take inclined sights.

2) It essentially consists of a small telescope with a level tube and graduated arc mounted on a horizontal axis.

3) It gives higher accuracy and more range of sights.

4) This telescope is fitted with a stadia diaphragm and can be used as a tachometer also for computations of horizontal distance and vertical elevations.

## What is the method of Temporary Adjustment of the Dumpy level?

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Ans- The following are the different steps to be followed in a temporary adjustment

1. Selection of suitable position- A suitable position is selected for setting the level. From this position, it should be possible to take more number of observations without any difficulty and the ground should be fairly level and firm.

2. Fixing level of tripod stand- The tripod stand is placed at a required position with its leg well apart and press firmly into the ground. The level is fixed on the top of the tripod stand according to the fixing arrangement provided for that particular level. It should be remembered that the level is not to be set up at any station.

3. Approximate levelling by a leg of tripod stand- The foot screw is brought to the centre of their run. Two legs of the tripod stand are firmly fixed into the ground .then the third leg is moved left or right in or out until the bubble is approximately at the centre of its run.

4. Perfect levelling by foot screw- As the longitudinal bubble is on the top of the telescope the latter is placed parallel to any pair of foot screw. (i.e. first position) and the bubble is brought to the centre by turning the foot screw equally either both inward and both outwards. The telescope is then turned through 900 and brought to a third foot screw and the bubble is brought to be centre by turning this foot screw clockwise or anticlockwise the telescope again brought to its original position and the bubble is brought to the centre.

5. Focusing the eyepiece- a piece of white paper is held in front of the object-glass and the eyepiece is moved in or out by turning it clockwise or anticlockwise until the crosshair can be seen clearly.

6. Focusing the object-glass- the telescope directed toward the levelling staff. Looking through the eyepiece the focusing screw is turned clockwise or anticlockwise until the graduation of the staff is distinctly visible and parallax is eliminated.

7. Taking the staff reading- finally, the level of the instrument is verified by turning the telescope in any direction.

## What are the personal and instrumental errors in levelling?

Ans- Instrumental Errors:

1) Error Due to Impact Adjustment

The essential adjustment of a level is the line of sight shall be parallel to the axis of the bubble tube. The error in the rod reading will be proportional to the distance and could be eliminated by balancing the backsight and foresight distances.

The error is likely to be cumulative, particularly in going up or down a steep hill, where all backlights’ are longer or shorter than all foresight unless care is taken to run a zigzag line.

2) Error Due to Sluggish Bubble

When the bubble is sluggish, it will come to rest in the wrong position, though it could creep back to the correct position while the sight is being taken. Such a bubble is a constant source of delay and aggravation. However, the error might be partially avoided by observing the bubble after the target has been sighted. The error is compensating.

3) Error in the Movement of the Objective Slide

In the case of external focusing instruments, if the objective slide is slightly worn, it may not move in a truly horizontal direction. From the short sights, the objective slide is moved out nearly its entire length, and the error is, thus, more.

4) Rod Not Standard Length

Incorrect lengths of divisions on a rod cause errors similar to those resulting from the incorrect marking on a tape. The error is systematic and can be directly proportional to this difference in elevation.

Personal Errors:

1) Mistakes in Manipulation

These include mistakes in setting up the level, imperfect focusing on the eye-piece and of objective, errors in centring the bubble and failure to watch it after each sight, and errors because of resting the hands-on tripods or telescope.

2) Mistake in Rod Handling

If the rod isn’t in plumb, the reading taken will be too great. The error varies directly with the magnitude of the rod reading and directly as the square of the inclination. In running a line of levels uphill, backsight readings are likely to be increased more than foresight from this source along with the evaluation of a bench -Mark on top will be too great.

3) Mistake in Reading the Rod:

1. Error in reflection

2. The common mistakes in reading the rod are :

4) Errors in Sighting

The error is caused when it is difficult to tell when the crosshair coincides with the centre of the target in a target rod and to determine the exact reading that the cross-hair appears to cover in the case of this self-reading rod. This is an accidental error the magnitude of which depends upon the coarseness of this cross-hair, the type of rod, the form of target., atmospheric conditions, length of sight and the observer.

## Explain the method of Profile levelling.

Ans- The profile drawing is basically a graph of elevations, plotted on the vertical axis, as a function of stations, plotted on a horizontal axis. A gridded sheet called profile paper is used to plot the profile data from the field book. All profile drawings must have a proper title block, and both axes must be fully labelled with stations and elevations.

The elevation or elevation scale is typically exaggerated; that is, it is ‘stretched’ in comparison to the horizontal scale. For example, the vertical scale might be 10 times larger. The horizontal line at the bottom of the profile does not necessarily have to start at zero elevation.

Cross sectioning levelling is a method in profile levelling. The term cross-section generally refers to a relatively short profile view of the ground, which is drawn perpendicular to the route centerline of a highway or other types of linear projects.

## What are the Fundamental lines of Dumpy Level and what is the relationship between them?

Ans- The Fundamental lines of Dumpy Level are:

a. Line of collimation

b. Axis of the telescope

c. Axis of the bubble tube

d. Vertical axis

Their relationships:

a. The line of collimation should be parallel to the axis of the bubble tube.

b. The line of collimation should coincide with the axis of the telescope.

c. The axis of the bubble tube should be perpendicular to the vertical axis.

## Calculate the reduce level by Rise and Fall method on a continuous sloping ground with four meter levelling staff at the common interval of 30m. 0.855 (on A),1.545,2.335,3.115,3.825,0.455,1.380,2.055,2.855,3.455,0.585,1.015, 1.850, 2.755,3.845 (on B); The reduced level of A was 380.500. Make the entries in a level book and apply the usual checks. Determine the gradient of AB.

Ans-
 Station BS IS FS Rise Fall RL Remarks A o.855 380.500 BM 1 1.545 0.690 379.810 2 2.335 0.790 379.020 3 3.115 0.780 378.240 4 0.455 3.825 0.710 377.530 C.P 1 5 1.380 0.925 376.605 6 2.055 0.675 375.930 7 2.855 0.800 375.130 8 0.585 3.455 0.600 374.530 C.P 2 9 1.015 0.430 374.100 10 1.850 0.835 373.265 11 2.755 0.905 372.360 B 3.845 1.090 371.270

Summation BS= 1.895

Summation FS= 11.125

Summation Fall= 9.23

Arithmetic Check:

Summation BS - Summation FS = 1.895 – 11.125 = -9.23

Summation Rise – Summation Fall = 0 – 9.23 = -9.23

Last RL – First RL = 371.270 -380.500 = -9.23

Gradient of AB = 9.23/360 = 1/39.00325 Falling

## Detect the Local attraction at stations and correct the bearings of lines of a traverse ABCDEA. Also, calculate the included angles.

Ans-

 Line FB BB Difference Correction Corrected Difference FB BB FB BB AB 59° 239° 180° 59° 239° 180° BC 139° 30’ 317° 177° 30’ +2° 30’ 139° 30’ 319°30’ 180° CD 215° 15’ 36° 30’ 178° 45’ +2° 30’ +1° 15’ 217° 45’ 37°45’ 180° DE 208° 29° 179° +1° 15’ +0° 15’ 209° 15’ 29°15’ 180° EA 318° 30’ 138° 45’ 179° 45’ +0° 15’ 318° 45’ 138°45’ 180°

Angle A = 138° 45’ - 59° = 79° 45’

Angle B = 239° - 139° 30’ = 99° 30’

Angle C = 319°30’ - 217° 45’ = 101° 45’

Angle D = (37°45’ - 209° 15’) + 360 = 188° 30’

Angle E = (29° - 318° 30’) + 360 = 70° 30’

Angle (A+B+C+D+E) = 79° 45’ + 99° 30’ + 101° 45’ + 188° 30’ + 70° 30’ = 540°

= (2n – 90) * 90

(n=5)

Stations C, D and E are affected by local attractions.

*Self Typed
*Source- Books, Internet, Self-Analysis

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