Understanding Key Surveying Terms

Before starting, familiarize yourself with these essential terms:

  • BS (Backsight) – A staff reading taken at a known elevation point.

  • FS (Foresight) – A staff reading taken at a new point to determine its elevation.

  • RL (Reduced Level) – The elevation of a point relative to a known benchmark.

  • TBM (Temporary Benchmark) – A reference point with a known RL used for leveling.

  • HI (Height of Instrument) – The height of the instrument line of sight above a known point.

Procedure to Transfer RL from One TBM to Another

1. Setting Up the Instrument

Set your leveling instrument at Station A. Ensure the staff held vertically over TBM-1 and TBM-2 is visible from this position.

2. Taking Initial Readings

Record a Backsight (BS) reading over TBM-1. Then, take a Foresight (FS) reading over TBM-2.

3. Shifting the Instrument

Move the instrument to Station B. After proper leveling, take a BS reading on TBM-2 and an FS reading on TBM-3. Never move the staff on TBM-2 before both readings are taken.

4. Using an Assumed RL for TBM-1

If the RL of TBM-1 isn’t known, assume it as 100.000. This assumption allows accurate calculations for subsequent TBMs.

RL Calculation Formula

  • HI = RL of TBM + BS

  • RL of new point = HI – FS

Apply these formulas at each station to find RL values of new TBMs.

Step-by-Step Example with Real Data

Let’s work with this set of leveling readings:


0.695, 1.250, 1.675, 0.470

The instrument is shifted after the 2nd reading.

Assigning Readings

  • 1st and 3rd readings = BS = 0.695 and 1.675

  • 2nd and 4th readings = FS = 1.250 and 0.470

Assumed RL of TBM-1

We begin with:

  • RL of TBM-1 = 100.000

Now calculate:

HI at Station A

  • HI = RL + BS

  • HI = 100.000 + 0.695 = 100.695

RL of TBM-2

  • RL = HI – FS

  • RL = 100.695 – 1.250 = 99.445

HI at Station B

  • HI = RL of TBM-2 + BS

  • HI = 99.445 + 1.675 = 101.120

RL of TBM-3

  • RL = HI – FS

  • RL = 101.120 – 0.470 = 100.650

Final RL Results

  • TBM-1 RL = 100.000 (assumed)

  • TBM-2 RL = 99.445

  • TBM-3 RL = 100.650

Important Tips for Accuracy

  • Always take BS and FS without moving the staff.

  • Ensure the instrument is correctly leveled at each station.

  • Use consistent and clear note-keeping for all readings.

  • Never skip readings between shifts of the instrument.

Click Here To See How To Find Distance By Leveling Machine, Theodolite, And Tachometer At Construction Site?

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