Levelling MCQ Quiz - Objective Question with Answer for Levelling - Download Free PDF

Concept:

True difference is given by the following formula:

TD = (δh_PQ when instrument is at Q + δh_PQ when instrument is at P)/2

where,

TD = True Difference

δh_PQ = Apparent difference in elevation between P and Q

Calculation:

When the instrument is at P, apparent height difference = 2.123 - 1.543 = 0.58

When the instrument is at Q, apparent height difference = 1.750 - 1.121 = 0.629

So,

\(TD = \frac{{0.629 + 0.58}}{2} = 0.6045\)

Now, doing the adjustment,

When the instrument is at Q, staff reading at Q = 1.750

So, Adjusted reading = 1.750 - TD = 1.750 - 0.6045 = 1.1455 = 1.146 m

Explanation:

1. Reciprocal levelling is a surveying technique used to accurately measure the difference in elevation between two distant points, especially when direct line-of-sight measurements are obstructed or when high precision is needed.

2. This technique involves taking readings at both ends of a line and using the average to eliminate errors caused by earth's curvature and atmospheric refraction.

3. By observing the same points from both directions, the inherent errors (like refraction) are effectively cancelled out. The approach minimizes the impact of errors that might otherwise accumulate over long distances.

Additional InformationLongitudinal Sectioning:

1. Definition: Longitudinal sectioning refers to the vertical cross-section along the length of a structure or terrain.

2. Effect on Earth’s Curvature & Refraction: This method does not directly address the issues of earth's curvature or refraction. It is primarily used to represent a profile along the length, which may involve elevation or slope measurements but does not involve compensating for curvature or refraction.

Profile Levelling:

1. Definition: Profile levelling involves measuring the heights of points along a line, typically following the contour of the ground or a specific path.

2. Effect on Earth’s Curvature & Refraction: While profile levelling is used to measure changes in elevation, it doesn't directly compensate for the earth’s curvature or atmospheric refraction. The measurements are typically taken with a level instrument, but it doesn’t specifically correct for these factors over long distances.

Cross Sectioning:

1. Definition: Cross sectioning involves taking vertical slices or sections across a terrain at specific intervals, usually perpendicular to a central line (e.g., a road or railway line).

2. Effect on Earth’s Curvature & Refraction: Cross sectioning is used to understand the profile of the terrain in relation to a central line but does not address the curvature of the earth or the refraction effect in its standard form. It's a method used to study the shape and gradient of the ground.

Explanation:

Tachometer:

(i) Tacheometer is nothing but a transit theodolite fitted with a stadia diaphragm.

(ii) The stadia diaphragm generally consists of two hairs, one above and one below and equidistant from a central horizontal hair.

Systems of Tacheometric measurements:

(a) Stadia system

(b) Tangential system

(c) Subtense bar system

(a) Stadia system:

(i) In the stadia system of tacheometry of, the tacheometer is set up station A and staff at station B.

(ii) The staff intercept between the upper and lower stadia is measured along with vertical angle θ made with horizontal.

(iii) The horizontal distance D between the instrument station A and staff station B and the difference of elevation between stations A and B are determined from the staff intercept (s) and the vertical angle.

(iv) Refraction correction may occur during measurement.

(b) Tangential system:

(i) Here in this system of tacheometry, the hairs are not at all required.

(ii) This method can even be used when telescope is not provided with a diaphragm.

(iii) The staff had two targets at a fixed distance (s) apart.

(iv) The vertical angles θ1 and θ2 are measured to the two targets.

(v) Now these vertical angles and the fixed distance on staff are used to determine the horizontal distance and difference of elevation.

(C) Subtense bar system:

(i) Here in this system of tacheometry, a bar of fixed length is used which is known as subtense bar as shown in fig.

(ii) The subtense bar has two targets at the ends at a fixed distance (s) apart.

(iii) The horizontal angle subtended between the instrument station A and the two targets on the subtense bar is measured.

(iv) In subtense bar system less chance of refraction error because the line of sight does not change the path 

Types of leveling:

Profile leveling 

• Profile leveling is a method of surveying that has been carried out along the central line of a tract of land on which linear engineering work is to be constructed/ laid.
• The operations involved in determining the elevation of the ground surface at small spatial intervals along a line are called profile leveling.
• The route along which a profile is run may be a single straight line, as in the case of a short sidewalk; a broken line, as in the case of a transmission line or sewer; or a series of straight lines connected by curves, as in case of a railroad, highway or canal.

Fly leveling 

Fly leveling is conducted when the benchmark is very far from the workstation. In such a case, a temporary benchmark is located at the work station which is located based on the original benchmark. Even it is not highly precise it is used for determining approximate level.

Differential leveling: 

It is the method of leveling to determine the elevation of points located at some distance apart or determine the elevation difference between two points or establish benchmarks. The method is used in order to find the difference in elevations between two points: (i) if they are far apart, (ii) if the difference in elevation between two points is too great and (iii) if there are obstacles intervening.

Reciprocal leveling: 

When it is not possible to locate the leveling instrument in between the intervisible points, reciprocal leveling is performed. This case appears in the case of ponds or rivers etc. In the case of reciprocal leveling, the instrument is set nearer to the 1st station and sighted towards the 2nd station.

Explanation;

Dumpy Level:

• It is an optical surveying levelling instrument consisting of a telescope tube firmly secured in two collars fixed by adjusting screws to the stage by the vertical spindle.
• The telescope of the dumpy level can rotate only in a horizontal plane.

Important Points Auto Level:

• It is an optical instrument used to establish or verify points in the same horizontal plane.
• It is used in surveying and building with a vertical staff to measure height differences.
• It does not require any protection from the sun.

                              Tilting Level:

• It consists of a telescope attached to a level tube which can be tilted within a few degrees in the vertical plane by a tilting screw.
• It is mainly designed for precise levelling work.

Wye Level:

• The essential difference between the dumpy level and the wye level is that in the former case, the telescope is fixed to the spindle while in the wye level, the telescope is carried in two vertical wye supports.
• The wye support consists of curved clips. The clips are raised, and the telescope can be rotated in the wyes, or removed and turned end for end.

Explanation:

Leveling staff:

• It is a self-reading graduated wooden rod having a rectangular cross-section. The lower end of the rod is shod with metal to protect it from wear and usually point of zero measurements from which the graduations are numbered.
• Staff are either solid (having a single piece of 3-meter height)  or folding staff (of 4-meter height into two or three pieces)
• The least count of a leveling staff is 5 mm.
• Leveling used with a leveling instrument to determine the difference in height between points or heights of points above a vertical datum.

Important Points

Explanation:

Two peg test:

(i) This method is either for an optical or digital level or a transit being used as a level. If this error is corrected with transit, it also improves the accuracy of its vertical angle readings.

(ii) It is performed to ensure that line of collimation of the telescope is parallel to the bubble tube axis.

Principle:

(i) The basic principle is that since the error in level readings results from the instrument not sighting exactly horizontally, is thus looking up or down at some angle and that this angle is the same whichever way it is sighting: if you place the instrument exactly midway between two-rod sightings, the vertical error reading on the rod is the same for each, thus the difference in reading between the rods will still give you an accurate elevation difference.

Explanation:

Given

We know

Height of Instrument (HI) = RL + Back sight (BS)

RL at any point = HI - Fore Sight (FS)

HI at P = 100 + 1.655 = 101.655 m

RL of Q = HI @ P - FS = 101.655 - (-1.500) = 103.155 m

HI @ Q = RL @Q + BS @ Q = 103.155 + (- 0.950) = 102.205

RL of R = HI @Q - FS @ Q = 102.205 - 0.750 = 101.455 m

Concept:

The height of the instrument is first established by means of taking sight towards the benchmark of known elevation.

1. Height of instrument = R.L of bench mark + Back sight reading
2. Elevation of station = Height of instrument - Fore sight reading

Calculation:

Given:

R.L of benchmark = 100.0 m, Back sight reading = - 2.105 m (staff is inverted)

Fore sight reading = 1.105

Height of instrument = 100 + (-2.105) = 97.895 m

Elevation of plinth = 97.895 - 1.105 = 96.790 m.

Curvature correction is given by:

C_c = 0.0785 × d²

Where,

d = distance in km = 1 km, C_c = Curvature correction in metres

C_c = -0.0785 × 1² = -0.0785

Noted staff reading = 1.565 m

Correct staff reading = Noted staff reading - C_c

Concept:

From the figure

\(tan\;\theta = Ground\;slope = \frac{{difference\;b/w\;length\;of\;staff\;and\;\left( {HI} \right)}}{{Horizontal\;distance}}\)

\(tan\theta = \frac{1}{{10}} = \frac{{4 - 1.5}}{x}\)

Calculations:

Given,

Height of instrument (HI) = 1.5 m

Length of staff = 4 m

Ground slope (down slope) = 1/10

\(tan\;\theta = \frac{{2.5}}{x} = \frac{1}{{10}}\)

x = 25 m.

Concept:

NOTE:

Reciprocal leveling eliminates the error due to collimation and error due to curvature of Earth completely, but the refraction depends upon the atmosphere which may change every minute.

​Reciprocal Leveling:

• ​This technique of leveling work is used to find the exact height difference or to find the exact RL(Reduce level) of the point by equalizing the distance when there is a large obstruction like a river, ponds, lakes, etc. in direction of the survey.

• It eliminates the following errors:

  i) error in instrument adjustments i.e error due to collimation

  ii) the combined effect of Earth's curvature and the refraction of the atmosphere

  iii) variation in the average refraction.

Explanation:

A relatively fixed point of known elevation above datum is called a benchmark.

Different types of benchmark are as follows:

A G.T.S. (Great Trigonometrical Survey) benchmark is a permanently fixed reference survey station (or point), having known elevation with respect to a standard datum (mean sea level). These are established all over India by Survey of India department with greater precision.

Permanent benchmark is intended to maintain its elevation without change over a long period of time with reference to an adopted datum, and is located where disturbing influences are believed to be negligible.

A temporary benchmark (TBM) is a fixed point with a known elevation used for level control during construction works and surveys. Nails in road seals, or marks on kerb & channel are commonly used as temporary benchmarks.

Concept:

According to H. I method or Collimation method,

H.I = R.L + B.S

R.L = H.I - (B.S/F.S)

Where,

H.I height of instrument, R.L = reduced = level, B.S = Backsight, F.S = foresight

Inverted staff reading

When a point for which R.L is to be determined is at a very high level above the line of sight, for example, the roof of a building, chajja, etc the leveling staff should be inverted such that the bottom of leveling staff should touch a point. It is taken as a negative reading.

Height from the base to bottom of inverted staff is kept = B.S - F.S

As staff is inverted, therefore reading of B.S is taken as negative.

Calculation

Given,

B.S = 1.32 m

RL of station = 115.385 m

F.S = -2.835

H.I = R.L +B.S

H.I = 115.385 + 1.32 = 116.705 m

Now, R.L bottom of the bridge deck,

R.L = H.I – F.S

R.L = 116.705 - (- 2.835) = 119.54 m 

Concepts:

Precise Leveling is used for establishing bench marks with high precision at widely distant points.

• It requires the use of highly modern instruments and greatest care in the field.  
• A high grade level equipped with tilting screw, stadia wires and coincidence level etc. and an invar precision leveling staff are commonly required.
• The parallax should be entirely eliminated by correct focusing.
•  The staff should be exactly vertical.  
•  The bubble should be exactly in the centre of its run at the time of taking readings.
•  Lengths of sights are limiting to about 100 m.
• The back sight and fore sight distances should be exactly equal. Stadia readings may be taken for this purpose.
• To avoid error due to settlement of tripod and staff, the back sights and the following fore sights should be taken in quick successions and the order of taking readings is interchanged at alternate set up i.e. at first setting, the back sight is observed first and then the fore sight while at the 2nd setting, the foresight is taken first and then the back sight and so on.

Rod readings are taken against the two horizontal hairs of the diaphragm. This is done in Tachometry Survey in which horizontal and vertical distances are taken.