Profile and cross-section levelling

Profile levelling (longitudinal sectioning) is levelling along a fixed line, such as the centre line of a road, canal, railway, or pipeline, to find the reduced levels of points along it at regular intervals (usually 10 m, 20 m, 30 m) and at every change of slope. The levels are plotted against chainage to get the longitudinal section (profile) of the ground.

Cross-section levelling is levelling along lines at right angles to the centre line at each chainage, to find the ground levels on both sides. Cross-sections show the ground profile across the alignment and give the width to be cleared and the volume of earthwork.

Field procedure

1. Peg the centre line at the required intervals and mark the chainages.
2. Set the level at a convenient point; take a BS on the BM and then IS on every peg along the centre line (the staff is held on the ground at each chainage and at breaks of slope); take FS on a change point when the instrument must be shifted.
3. For cross-sections, at each chainage set a line at right angles (with a cross-staff or prismatic square) and take staff readings at fixed offsets (e.g. 5 m, 10 m, 15 m) on the left and right, as IS.
4. Close the survey on another BM (or the first BM) and apply the arithmetic check and misclosure adjustment.

 Longitudinal section          Cross-section at ch. 120
 (RL vs chainage)                 left    CL    right
      ______                       \      |     /
  ___/      \___ ground              \____|____/
 -------------------- datum          15  5  0  5  15 m
 0   30  60  90 120

Uses in civil engineering

• To fix the formation level, gradient and depth of cutting/filling of a road, railway or canal.
• To compute earthwork quantities from the cross-sections (area and volume).
• To design the longitudinal gradient of sewers, water pipes and drains.
• To select the best alignment of roads and canals.
• To find the capacity of reservoirs and to plan bridge sites.