ELS_Levelling

What is the principle of levelling in surveying?

Levelling is the operation required to determine the heights of points on the surface of the Earth.

What equipment is necessary for levelling?

1. Level with tripod, 2. Levelling staff, 3. Chain/Tape, 4. Change plate, 5. Staff bubble.

What is the purpose of the telescope in a surveyor's level?

It forms a real image that is focused to lie in the same plane as the diaphragm.

How is parallax minimized in a surveyor's level?

By adjusting the eyepiece and focusing screw to eliminate parallax.

What does a compensating device do in an automatic level?

It corrects for residual mislevelment after the telescope is approximately levelled.

What is the construction material of a standard levelling staff?

Aluminium alloy.

What is the accuracy of a precision levelling staff?

It has an accuracy to 0.1mm when used with a precise level.

What is a level line relative to Mean Sea Level (MSL)?

A curved line at a constant height relative to MSL.

Define horizontal line in relation to a level line.

It is tangential to the level line at a point and perpendicular to the direction of gravity.

What is Mean Sea Level (MSL)?

The mean of tidal heights over a period of time greater than 19 years.

What is Principal Datum Hong Kong (PDHK)?

A reference point for most works in Hong Kong, located 1.230m below MSL.

What is a Benchmark (BM) in surveying?

A point of known elevation above PDHK, typically established on strong vertical surfaces.

What is Temporary Benchmark (TBM)?

A benchmark constructed at specific on-site locations calculated from PDHK.

What does Reduced Level (RL) signify?

The height of a point relative to PDHK.

What is Backsight (BS)?

The first sight taken after the level is set up to a known height point.

What is Foresight (FS)?

The last sight before moving the level, taken to a required point's height.

What is Intermediate Sight (IS)?

Staff readings taken between Backsight (BS) and Foresight (FS).

What is a Change Point (CP)?

The staff position at which a Foresight (FS) and then a Backsight (BS) readings are taken.

What is the first step in field procedure during levelling?

Set up the level at a point where a Bench Mark (BM) can be observed.

How is the first reading (BS) taken?

Using the staff on a point of known Reduced Level (RL).

What method is used to calculate the Height of Collimation (HC)?

HC = Known RL + Staff Reading.

In the Rise and Fall Method, what indicates a 'rise'?

When the second reading is less than the first reading.

In the Rise and Fall Method, what indicates a 'fall'?

When the second reading is greater than the first reading.

What is the allowable misclosure in levelling?

± 5√n mm where n is the number of instrument positions.

When should levelling be repeated?

If the actual misclosure exceeds the allowable misclosure.

What is the importance of a bracket bubble in levelling?

It helps ensure that the staff is held vertically.

What does the term 'collimation error' refer to in levelling?

When the line of sight or collimation is not truly horizontal, causing errors in height measurement.

What is the two peg test used for?

To adjust for collimation error by comparing staff readings taken from two points.

What significant error is corrected during the two peg test?

The inclination error of the collimation line.

What are common errors in levelling equipment?

Collimation error, parallax error, and defects of the staff.

How does staff verticality affect measurements in levelling?

If the staff is not vertical, it can lead to inaccurate readings.

What should be considered when leveling on unstable ground?

Use a change plate to stabilize the setup.

What practice helps detect misclosure in levelling?

Start and finish at points of known height.

What is the role of curvature in levelling observations?

Curvature affects the accuracy of line of sight over long distances.

How does atmospheric refraction affect leveling?

It bends the line of sight downwards, affecting height calculations.

What method can avoid corrections for curvature and refraction?

Reciprocal levelling.

In reciprocal levelling, how is the height difference calculated?

H = ((a1 – b1) + (a2 – b2)) / 2.

What correction is typically applied for curvature?

c ≅ L² / 2R, where L is the distance and R is Earth's radius.

What combined correction for curvature and refraction is considered?

Combined correction = 0.0785 L² * (6/7).

How is the correction for curvature and refraction computed for a 200m distance?

Using the formula combined with the given coefficients.

What is the final step to check accuracy in levelling work?

Ensure that the sum of Back Sight (BS) minus Foresight (FS) equals the difference in levels.