Study notes — CPCCOM3006 Carry out levelling operations

What this unit is about

Overview

This unit is about taking accurate levels on a construction site — setting up a levelling instrument, reading a staff, booking the readings in a field book, and calculating reduced levels using both the rise and fall and height of collimation methods. Get it right and slabs, footings, floors and ceilings finish at the heights the drawings call for; get it wrong and the mistake ends up set in concrete.

Element 1

Before you sight anything: plan and prepare

Read the job first. Work through the work instructions, drawings and specifications, and plan the sequence of the work — know which points need levels before you set up anything.
Plan for compliance. The job must sit inside WHS laws and regulations, environmental requirements, relevant Australian Standards, the manufacturer's instructions for your instrument, and your workplace's own procedures — including the SWMS if one applies to the task.
Select and check the gear. Level, tripod, staff, tapes, pegs — and your PPE. Check everything for serviceability before you leave the shed. Report faults and tag out damaged gear; never "make do" with a knocked-about instrument.
Fit PPE correctly. Hard hat, hi-vis and safety boots as the site requires — worn and adjusted properly, not just carried to the job.
Walk the site before setting up. Look for hazards and services — overhead power, trenches and excavations, moving plant, underground services — and put risk controls in place, including signage and barricades where your sight lines cross traffic or work areas.
Sort your materials. Work out what the task needs (pegs, stakes, string line, marking paint), calculate the quantities, handle them safely and position them ready for use.
Sort your team. Levelling is a two-person job. Confirm who runs the instrument and who holds the staff, and agree your verbal calls and hand signals before you start — on a noisy site, you often can't hear each other across the distance.

Element 2

The rise and fall method

One person sights, one holds the staff — and the booking sheet does the maths by comparing each reading with the one before it.

Test before you trust. Set up the instrument and check it works accurately — the two-peg test confirms an automatic level's line of sight is genuinely horizontal.
Start from a datum. Set up or locate a benchmark or temporary benchmark (TBM) with a known reduced level (RL). Every other level on the job is calculated from it.
Know your three sights. The first reading after every set-up is a backsight (BS); the last reading before you move the instrument is a foresight (FS); everything in between is an intermediate sight (IS). Book each in its own column, in order, as you go.
Change points let you cover ground. Take a foresight to a solid, stable change point, move the instrument, then take a backsight to the same point — both readings go on the same line of the field book, with the change station noted.
Rise or fall? Compare each staff reading with the previous one from the same set-up: a smaller reading means the ground rose; a larger reading means it fell. New RL = previous RL + rise, or − fall.
Prove the arithmetic. Apply the three-check method before you call the page finished.

The instrument's line of sight is horizontal. Datum RL + backsight sets the height of the line; every other RL comes back down from it.

Worked example — rise and fall booking
BS	IS	FS	Rise	Fall	RL	Remarks
1.500					100.000	TBM (datum)
	1.250		0.250		100.250	Stn A
2.100		1.850		0.600	99.650	Stn B — change point (FS + new BS, same line)
		1.400	0.700		100.350	Stn C
3.600		3.250	0.950	0.600	100.350	Totals for the checks

The three-check method

ΣBS − ΣFS = 3.600 − 3.250 = 0.350

Σrises − Σfalls = 0.950 − 0.600 = 0.350

Last RL − first RL = 100.350 − 100.000 = 0.350 ✓

All three must agree exactly. If they don't, the arithmetic on the page is wrong — find it before anyone builds off your figures.

Element 3

The height of collimation method

Same fieldwork, different arithmetic: work out the height of the instrument's line of sight once, then subtract every reading from it.

Field procedure is identical. Set up and test the instrument and check it's within tolerance; set up or locate the datum; take backsights, intermediate sights and foresights exactly as before.
Establish the height of collimation (HPC). Take the backsight to the datum and add it to the datum's RL: HPC = 100.000 + 1.500 = 101.500. That's the RL of the line of sight itself.
Every station comes straight off the HPC. RL = HPC − staff reading. Big reading, low ground; small reading, high ground. Stn A: 101.500 − 1.250 = 100.250. Stn B: 101.500 − 1.850 = 99.650.
Moving the instrument means a new HPC. New HPC = RL of the change point + the new backsight: 99.650 + 2.100 = 101.750. Record the change station in the field book. Stn C: 101.750 − 1.400 = 100.350 — the same answers as rise and fall, as they must be.
Book as you go. Every reading, every HPC and every RL goes into the field book at the time — never written up later from memory.
Check it. Verify the page with the collimation arithmetic check and the three-check: ΣBS − ΣFS must equal last RL − first RL.
When to use which. HPC is faster when there are many intermediate sights (set-out work, shooting a slab); rise and fall gives the fuller line-by-line arithmetic check. You need to be fluent in both.

Same readings — height of collimation booking
BS	IS	FS	HPC	RL	Remarks
1.500			101.500	100.000	TBM (datum)
	1.250			100.250	Stn A
2.100		1.850	101.750	99.650	Stn B — change point, new HPC
		1.400		100.350	Stn C
3.600		3.250		100.350	ΣBS − ΣFS = 0.350 = last − first ✓

Element 4

Distances from stadia lines

Look closely at the crosshairs. Above and below the main horizontal crosshair sit two short stadia lines. They're a built-in distance measurer.
Read the staff at both stadia lines. Distance from instrument to staff = (top reading − bottom reading) × the instrument's multiplying constant — 100 on most levels. Check your instrument's identified factor; don't assume.
Example: top stadia 1.845, bottom stadia 1.545 → interval 0.300 × 100 = 30 m.
Record it all. Book the stadia readings and the calculated distance alongside your levels — not just the answer.

(1.845 − 1.545) × 100 = 30 m from instrument to staff

Knowledge evidence, distilled

Know this cold

Rules and quality: the Australian Standards relevant to levelling operations, your workplace's quality policies and standards, and the project's own quality requirements — how accurate your levels must be, and how results are recorded and signed off.
Safety: the safety requirements for levelling work, and who holds the responsibility and authority for site isolation and traffic control when your sight lines or set-out cross traffic or plant routes.
Reading the job: construction plans, symbols and terminology; how to interpret engineering drawings and sketches for levelling work; and how set-out for levelling tasks is carried out on site.
The toolkit: spirit levels, straight edges, automatic/optical levelling devices, laser levels and the humble water level — what each is for and when you'd reach for it.
Checking the gear: the two-peg test for an automatic level; reverse (end-for-end) readings to check a spirit level. Know both procedures and the tolerances they must sit within.
Types of reading: datum, backsight, foresight, intermediate sight, plumb, and negative (inverted) readings for points above the line of sight, like a soffit — booked as negatives.
The core activities: establishing approximate distances and transferring reduced levels; locating hazards and services; ground levels at critical set-out points; heights for installing building components; floor and ceiling levels; slab or pad levels for steel columns or masonry piers; shooting levels for concrete slabs, excavations and footings; and calculating both rise and fall and height from collimation.
Care of instruments: cleaning; authorised operator servicing and minor replacements only; and monitoring, recording and reporting faults. A level that's been dropped goes back for service — not back in the box.

Learn from other people's mistakes

Where people slip up

"Backsight means the direction you're facing." No. The backsight is simply the first reading after any set-up, and the foresight the last before you move — regardless of which way the instrument points. Everything else is an intermediate sight.

Fix: think first-and-last, not front-and-back.

Skipping the two-peg test. A centred bubble does not guarantee a horizontal line of sight — a knocked instrument can look level and still read wrong at every station.

Fix: two-peg test before you trust the instrument; reverse readings before you trust a spirit level. Report and record anything outside tolerance.

Booking a change point on two lines. The foresight to the change point and the backsight from the new set-up are two readings of the same point — split them onto separate lines and the whole calculation falls apart.

Fix: same point, same line: FS and BS side by side, change station noted.

Treating the three-check as proof the survey is right. It only proves your arithmetic. A misread staff or a wrong staff hold sails straight through it.

Fix: close the run back onto the datum and compare — the misclose must sit within the project's tolerance.

A staff held off plumb. Any lean makes the reading bigger than the true value, so the calculated RL comes out too low — every time.

Fix: use the staff bubble, or rock the staff slowly towards and away from the instrument and book the lowest reading.

The language of levelling

Mini-glossary

Datum / benchmark: A fixed point of known reduced level from which all other levels on the job are calculated. A TBM (temporary benchmark) is one you establish for the project.
Reduced level (RL): The height of a point above the datum, in metres — the number the drawings and your field book trade in.
Backsight (BS): The first staff reading taken after each instrument set-up — usually to a point of known RL.
Intermediate sight (IS): Any reading between the backsight and the foresight from the same set-up.
Foresight (FS): The last reading taken before the instrument is moved.
Change point (CP): A solid, stable point that receives a foresight from one set-up and a backsight from the next — the link between instrument positions.
Height of collimation (HPC): The RL of the instrument's horizontal line of sight: datum (or change point) RL + backsight.
Rise / fall: The change in ground height between consecutive readings: a smaller subsequent reading is a rise, a larger one a fall.
Two-peg test: The field check for an automatic level, comparing readings between two pegs from mid-point and off-centre set-ups to expose collimation error.
Stadia lines: The two short horizontal lines in the reticle used with the instrument's multiplying constant (usually 100) to calculate distance to the staff.
Plumb: Truly vertical. The staff must be plumb when read, or every reading is high.
Inverted (negative) reading: A reading taken with the staff held upside down against a point above the line of sight, such as a soffit — booked as a negative value.
Misclose: The difference between a known RL and the value you get when a level run closes back onto it — the real test of a run, checked against tolerance.

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