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�INTRODUCTION TO BASIC SURVEYING

Dr Adewale Abimbola, FHEA, GMICE.

www.edulibrary.co.uk

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Aim & Objectives

Aim: Introduction to Basic Surveying

Objectives: At the end of the lesson, the students should be able to:

  • Read levelling staff.
  • Calculate the horizontal distances using stadia tacheometry.
  • Identify the location of the back sight (BS), intermediate sight (IS), foresight (FS), and the change point (CP).

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Levelling in Surveying: An Overview

  • Levelling is a comprehensive term encompassing diverse methodologies for determining elevations and variations in elevation across different points. This process is crucial in surveying and construction.

Procedure:

  • Elevation Reference:
    • Upon determining the elevation of a specific point, it becomes a reference for establishing the elevations of other points in the survey.
  • Setting Up Vertical Rods:
    • Points A and B are selected.
    • Vertical rods are erected at both points, and a levelling instrument is positioned between them.
  • Height Measurement:
    • The difference in height (∆H) between points A and B is measured using the rod (staff) readings.

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Figure 1. Levelling procedure

Significance of Levelling

  • It is foundational in surveying, providing a standardised approach to ascertain height differentials and ensuring accuracy in elevation calculations, which is vital for precise construction and engineering applications.

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Applications of Levelling in Construction/Civil Engineering

  • Construction of Infrastructure: Levelling is used to establish grade lines during the construction od various infrastructure projects such as highways, railroads, canals, sewers, and water supply systems.
  • Construction Layout: For construction projects, levelling is pivotal in laying out structures according to predetermined elevations. This ensures that construction aligns precisely with the planned design.
  • Drainage Analysis: In the investigation of drainage characteristics of an area, levelling provides essential data. It assists in understanding the flow of water and designing drainage systems that effectively manage water runoff.
  • Earthworks Calculation: Levelling plays a crucial role in calculating the volume of earthworks and other materials required for construction projects. This helps in effective project planning and resource allocation.

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Ordnance Survey Benchmarks (BMs)

Ordnance Survey Benchmarks (BMs) serve as survey marks created by the Ordnance Survey to establish height above Ordnance Datum, aiding in precise elevation determination.

Datum Definition:

  • The ordnance datum in the UK, denoting 0 meters on maps, is the mean sea level (MSL) measured at Newlyn in Cornwall between 1915 and 1921. Before 1921, it was referenced to MSL at the Victoria Dock, Liverpool.

Height Determination:

  • The accurate height of one BM allows for the determination of the next BM's exact height by measuring the difference in elevations through spirit levelling.

Symbol on Maps:

  • Ordnance Survey benchmarks are represented on maps by a specific symbol (Fig. 2), approved after 1854. Typically, it appears as an incised horizontal bar with a broad arrow immediately below, with the bar's height precisely determined through spirit levelling.

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Figure 2. Benchmark symbols

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Key Concepts in Basic Surveying

Reduced Level:

  • The reduced level of a point signifies its height above a reference plane, providing a crucial measure in levelling surveys.

Height:

  • Height is defined as the vertical distance from a level reference plane or surface to a specific point, aiding in precise elevation determination.

Closure Error:

  • The disparity between the elevation determined through levelling surveys and the known elevation of a benchmark, indicating the accuracy of the levelling operation.

Figure 1. Levelling procedure

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Automatic Level and E-type Levelling Staff

Figure 3. Automatic Level

Figure 4. E-type Levelling Staff

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Automatic Level and E-type Levelling Staff

Automatic level and levelling staff can also be used to measure horizontal distances. This method is known as stadia tacheometry.

Horizontal distance = 100 X Distance recorded on the levelling staff between the top and the bottom stadia hairs.

Worked Example: if the top stadia hair reads 1.235 m and the bottom hair reads 1.105 m, then the direct horizontal distance between the instrument and the staff positions would be:

Horizontal distance = 100 X (1.235 m – 1.105 m)

= 100 X 0.13 m

= 13 m

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Self-assessment Task 1

  1. Make a list of the staff readings ‘a’ to ‘f’ in the figure below.

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Self-assessment Task 1

2) Calculate the distance for the optical level staff readings below:

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Levelling – Booking Readings

  • The first reading is taken from a point of known reduced level (either an OSBM or TBM) and this is called a BACKSIGHT (BS).
  • Any reading which follows is known as an INTERMEDIATE SIGHT (IS).
  • The last reading from the instrument position will be a FORESIGHT (FS).
  • If the instrument moves and the staff stays in the same place, this point where the staff stays is known as a Change Point (CP).

Figure 6. Levelling procedure. (Sham, 2017)

Figure 5. Staff readings. (Expert Civil, 2023)

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Self-assessment Task 2

  • Identify the location of the back sight (BS), intermediate sight (IS), foresight (FS), and the change point (CP) in figure 7.

Figure 7. Levelling procedure. (Boeingconsult, 2016)

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Reference/Bibliography

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