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?