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How to calculate the RL of temporary benchmark on the site

How to calculate the RL of temporary benchmark on the site.

In this Article I will Discuss about the topic of How to calculate the RL of temporary benchmark on the site.

Let us now go through the procedure of shifting And finding the RL of the temporary benchmark on the Construction site.


Full form.

BS      = Backsight

FS      = Foresight

R L    = Reduced level

TBM = Temporary benchmark

HI     = Height of instrument

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While the reading procedure, observe the above given drawing (for each step) to understand the concept.

1 – We have to set the instrument at the station A. The location of the instrument station should be such that the staff held over the TBM – 1 and TBM – 2 should be visible.

2 – Note down the BS reading over the staff held over TBM – 1 and FS over the staff positioned over the TBM – Two.

3 – Shift the instrument position from the station A to station B. After leveling and setting the instrument, take BS reading over TBM – two and FS reading over the TBM – three.

In this process, the shaft held over the TBM – two should not be lifted or moved before observing FS and BS readings.

4 – If the RL of TBM – One is not known, consider it as 100.


HI at the station A   = [RL of TBM-One + BS from station A]

RL of TBM – Two      =  [HI at station A – FS from station A]


HI at the station B = [RL of TBM-two + BS from the station B]

RL of TBM-three    =  [HI at station B – FS from the station B]

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In this way, u can transfer the TBM to the desired locations on construction site.

Let us consider an example to understand the procedure.


The following readings were taken with the leveling instrument to transfer the TBM on the site.

0.695, 1.250, 1.675, & 0.470. The instrument is shifted after Second reading. The RL of the 1st reading taken over TBM-one is 100. Find the RL of the TBM-Two and TBM – three.



As u can observe in the above drawing, the instrument is shifted after the Second reading.

So, The 1st and 3rd readings        👉 BS   👉 0.695 and 1.675.

Second and 4th readings        👉  FS  👉 1.250 and 0.470.

Please remember. While Surveying, the 1st reading becomes BS and the last reading becomes FS. 

HI at the station A 

= [RL of TBM-One + BS from the station A]

= [100 + 0.695]


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1 – RL of TBM- Two

=  [HI at station A – FS from the station A]

= [ 100.695 – 1.250]



HI at station B 

= [RL of TBM-two + BS from the station B]

= [99.445 + 1.675]


2 – RL of TBM – three

=  [HI at station B – FS from the station B]

= [101.12 – 0.470]


Please Visit My YouTube Channel For Land Surveying Basic Knowledge.
Raja Junaid Iqbal

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About Raja Junaid Iqbal

Hi My name is Raja Junaid Iqbal and i am Land Surveyor by Profession but I've specialized in the field of Q,S and land development. As Professional Surveyor I engaged my self in all over the Gulf since 14 years.

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