Profile levelling; balancing of sight

Profile levelling (longitudinal sectioning) is levelling along a given line, such as the centre line of a road, railway, canal or pipeline, to determine the elevations of the ground at regular intervals and at changes of slope, in order to draw the longitudinal section of the ground along the line.

Procedure

1. Peg the centre line at equal intervals (10, 20 or 30 m) and note the chainage of each peg and of the breaks of slope.
2. Set up the level on firm ground within about 100 m of a BM and take a BS on the BM.
3. Hold the staff on each peg within reach along the line and take an intermediate sight (IS); take an FS on a change point when the level has to be shifted, and a BS on that point from the new position.
4. Close the levelling on a BM (or the starting BM) to check the work.
5. Reduce the levels (HI or rise and fall method), apply the arithmetic check, and distribute any closing error.
6. Plot the RLs against the chainage (with the vertical scale larger than the horizontal) to get the profile.

 RL
  |   ____
  |__/    \__     ___  <- ground profile
  |          \___/
  +------------------- chainage

Balancing of sight

Balancing of sights means keeping the backsight distance equal to the foresight distance from each instrument position.

• The instrument is placed midway between the BS and FS points as far as possible.
• The error due to the line of collimation not being horizontal is proportional to the sight distance, and the errors due to curvature of the earth and atmospheric refraction are proportional to its square. For equal sights these errors are equal in the BS and FS readings and cancel when the difference is taken.
• When equal distances cannot be kept at each setup (for example on steep ground), the sum of the BS distances is kept equal to the sum of the FS distances. This still cancels the collimation error; the curvature and refraction errors are negligible for ordinary sight lengths.

It is also followed in fly levelling and in differential levelling generally.