Surveying Techniques and Levelling Operations

Leveling Unit 3

• Leveling is the process of determining the relative heights of points on or below the Earth's surface. It focuses on measurements in the vertical plane.
• Leveling is the overall term for various processes used to determine elevations or differences in elevation.
• It involves measuring vertical distances using direct or indirect methods to ascertain elevations.

Leveling Terminologies

• Datum: An imaginary level surface to which all elevations are referenced (often sea level).
• Elevation: The vertical distance from a datum to a point or object.
• Bench Mark (BM): A permanent, natural or artificial object with a marked point whose elevation relative to a datum is known or assumed.
• Back Sight (BS): The initial reading taken after setting up the level.
• Fore Sight (FS): The last reading taken before moving the level; it's also a reading to a point whose height is needed.
• Intermediate Sight (IS): Staff readings taken between BS and FS (medium readings).
• Change Point (CP) or Turning Point (TP): Staff position where FS and then BS readings are taken.
• Reduced Level: Calculated elevation of a point above or below sea level; also called reduced height.

Primary Objective of Leveling

• Determine the elevations of given points relative to a datum.
• Establish points with a specified height above or below a datum line.

Uses of Leveling

• Setting the base level for buildings.
• Creating contour maps to determine sites for reservoirs, catchment areas, and alignments of roads, railways, canals, or sewage lines.
• Determining altitudes of points on a hill or finding reduced levels of points above or below the Earth's surface.
• Calculating the capacity of a reservoir.

Create Profile View Maps (Profile Leveling)

• Used to understand the shape of the ground surface.
• Measurements are taken along the center of the road, at regular intervals (e.g., 20m, 30m).

Create Cross Section Maps (Cross Section Leveling)

• Used to determine the undulations of the ground surface across the road.
• Measurements are perpendicular to the center line of the alignment, at regular intervals (e.g., 20m, 40m).

Types of Leveling

• Simple Leveling: A method for measuring elevation differences between two points that are close together and have no obstacles. The level instrument is set up at the midpoint allowing direct vision of both points.
• Differential Leveling: Used when points are far apart, the difference in elevation is large, or there are obstacles. An instrument is moved from station to station to determine elevations.

Theory of Differential Leveling

• Techniques for finding elevations: Leveling, Total Station Technique, GPS Vertical Position Technique, Remote Sensing (Lidar or Aerial photography).

Types of Level Instrument

• Dumpy Level
• Tilting Level
• Automatic Level
• Digital Level

Digital Level

• An automatic level with normal optical leveling capabilities and a graduated rod.
• Operates using electrical measurements; rod graduations use barcodes.
• Can get distance and elevation readings with a button press.
• Data is stored and transferable.

Automatic Level

• Uses a compensator to maintain a horizontal line of sight, even when the telescope is tilted.
• Bubble-based system centers itself (or nearly so), so the compensator maintains the horizontal line of sight.

Accessories

• Traditional Levelling Rod (Leveling Staff): Made of wood, metal, or fiberglass. The surveyor should study unfamiliar rods prior to use. Graduated in feet or meters,
• Foot Rod: Can be read to 0.01 ft.
• Metric Rod: Can be read to 0.01 meter with estimations for millimeters
• Levelling Rod (Barcode Rod): Rod with barcode graduated scale. Used with digital levels for distances up to 100m
• The rod should be plumb and held in the correct upright position for accurate measurement.
• Turning Level Plate: Part to help maintain a horizontal plane.
• Rod Bubble: Used to help maintain horizontal plane.

Suggestions for Rod Work

• Extend and clamp the rod properly and hold on firm ground.
• Hold the rod plumb for all readings.
• Use the same rod position for backsight and foresight at change points.
• If the rod is held nearby but not on the location, turn the face away from the instrument to avoid mistakes.

Two Peg Test

• Used to check accuracy of a level (optical or digital).
• Set up three points (A, C, B) in a straight line (A to C = C to B = 30m).
• Position the level at C and measure staff readings at A and B.
• The true difference in level is obtained, even if the instrument is slightly out of adjustment.

Two Peg Test 2

• Move the level to the higher end (B) and position it on a line with A, extending slightly beyond B.
• Read the staff readings at A and B again.
• If the instrument is adjusted, the B reading should equal the A reading plus the elevation difference measured in the first test.

Booking for Two Peg Test

• A table is used to record the data (eg. BS, IS, FS, Horizontal distance, reduced levels, etc.) with comments for each measurement.
• The difference between two measurements of the same point is used as checks to verify accuracy.

Rise and Fall Method

• Used for calculating RL's along a line by recording BS & FS and rise & fall.
• A table is used to record the data (eg. BS, IS, FS and others).
• Arithmetic checks help identify accuracy.

Height of Collimation Method

• Another approach for leveling calculations.
• Height of collimation is used to calculate the RL of a station by subtracting the staff reading from the current height of collimation.
• The height of collimation is updated when the level is repositioned.

Theory of Differential Leveling (Tide Gauge)

• A modern water level monitoring station with sensors to continuously record surrounding water levels.

Errors in Leveling

• Collimation Error: Level alignment issue.
• Parallax Error: Cross-hairs or image issue if not precisely focused.
• Error Due to Earth's Curvature: The line of sight deviates from horizontal as the distance increases.
• Error Due to Refraction: Light bending affects readings.
• Sight Length: 300 ft (1mm effect), keep sights under 150ft and equal lengths for backsights/foresights eliminate effects.