Levelling: height of instrument vs rise and fall, with the checks

A level gives a horizontal line of sight. Staff readings taken along it give the reduced level (RL) of each point relative to a benchmark.

Readings in a level book

• Back sight (BS): first reading after setting up, usually on a point of known RL.
• Fore sight (FS): last reading before moving the instrument.
• Intermediate sight (IS): anything in between.
• Change point (CP): a point with both an FS and the next BS.

Height of instrument (HI) method

HI = RL of point + BS RL = HI − (IS or FS)

Check: ΣBS − ΣFS = Last RL − First RL

Quick, good for many intermediate sights such as longitudinal sections and contouring. The check does not test the intermediate RLs.

Rise and fall method

Compare each reading with the one before: if the new reading is smaller, the ground has risen.

Rise = previous reading − current reading (if positive)

Checks: ΣBS − ΣFS = ΣRise − ΣFall = Last RL − First RL

Slower, but every RL is checked, so it suits precise work such as fixing benchmarks.

Worked example (HI method)

Point	BS	IS	FS	HI	RL
BM	1.525			101.525	100.000
A		2.110			99.415
CP1	0.985		1.870	100.640	99.655
B			2.240		98.400

ΣBS − ΣFS = 2.510 − 4.110 = −1.600

Last RL − First RL = 98.400 − 100.000 = −1.600 ✓

Common errors

Staff not vertical, bubble not centred, parallax, and reading the staff upside down. Keeping back sight and fore sight distances equal cancels collimation error and most curvature and refraction.

Our earthwork tool imports a level book in either format and runs these checks for you.