📐Bench Mark (BM) Shifting – Surveying Details

Bench Mark (BM) shifting is the process of transferring a known Reduced Level (RL) from an existing/old Bench Mark to a new location where a permanent reference point is required.

1. Basic Concept

BM (Bench Mark): A fixed point having a known RL/elevation.

BS (Back Sight): First staff reading taken on a known point after setting up the level.

FS (Fore Sight): Staff reading taken on the point whose RL is to be determined.

HI (Height of Instrument): Elevation of the line of collimation.

RL (Reduced Level): Elevation of a point with respect to the adopted datum.

2. Main Formula

HI = RL of Known BM + BS

Then:

RL of New BM = HI − FS

For example, if the old BM has RL = 100.000 m, BS = 1.250 m and FS = 0.850 m:

HI = 100.000 + 1.250 = 101.250 m

RL of New BM = 101.250 − 0.850 = 100.400 m

3. Differential Levelling

When the new BM is far away, several instrument setups and Change Points (CPs) are used. The FS on one setup and BS on the next setup are taken on the same stable change point.

A useful check is:

ΣBS − ΣFS = Last RL − First RL

4. Important Field Practices

✅ Keep the BM and change points stable and undisturbed.
✅ Keep BS and FS distances reasonably balanced to reduce errors.
✅ Check the level bubble before observations.
✅ Focus the eyepiece and objective properly to minimize parallax.
✅ Record readings immediately and verify calculations.
✅ Use a suitable datum and maintain consistent RL references.

Note: A BM should be established on a permanent, stable structure or approved survey monument—not on loose or temporary ground.

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