Setting out a slab with digital layout tools

Digital setting out has changed how a slab goes in. A tablet with an augmented reality overlay, a robotic total station or a GNSS rover can place the corners of a building on the ground faster and more consistently than a crew with tapes and string lines. The tools are good, and most builders who have used them would not go back. The trouble is that a digital layout is only as reliable as the reference it was built from, and the step that most often gets skipped is the one that checks the layout against that reference.

The problem usually shows up after the concrete has been placed. A slab sits a little off the boundary, a garage is out of line with the approved plans, or the internal walls are in the right place but the footings are offset from the centreline they were designed around. By then the cost is measured in demolition, redesign and a conversation with the certifier that nobody wanted to have. The layout was not wrong because the tool was bad. It was wrong because it was accepted without being tested.

This article is about that testing step. It covers how a digital layout relates to the surveyor’s marks and the approved plans, what to check before any concrete is ordered, how to record the check, and what to do when the layout and the reference do not agree. It is not a survey manual, and it does not replace a registered surveyor where the work requires one. It is written for the licensed builder who uses digital tools and wants to be confident that the tool is telling the truth.

A digital layout is a model of the approved plans, not the plans

Every digital setting out starts with a model. The model is built from the approved plans, and it is placed on the ground using a reference: a survey control point, a boundary mark, a benchmark or a set of marks the surveyor has placed. The layout tool then shows the crew where each corner, pier and footing should go. The accuracy of the result depends on three things: the approved plans in the model, the reference that the model is tied to, and the way the tool was set up to connect the two.

Each of those three can be wrong without anyone noticing. The model may have been built from a superseded revision of the plans. The reference may have been picked from the wrong set of marks. The tool may have been set to the wrong coordinate system, or the orientation may have been flipped so that the building faces the opposite way on the ground. None of these faults shows on the screen. The tablet looks confident, and the crew follows the line it shows, and the error is only found when someone measures against something independent.

The discipline is to treat the digital layout as a set of claims that need checking. Every claim should be tested against a second source before anyone relies on it for a pour. The second source is the surveyor’s marks and the approved plans, read directly rather than through the model.

Start with the right plans and the right marks

Before the tool is opened, confirm which set of plans is being used. The approved plans are the ones attached to the consent or the certificate, and they carry a revision that should match the model exactly. If the architect has issued a later revision, check whether it has been approved, and whether the change affects the position of the slab or the footings. A model built from an earlier set will set out a building that the certifier has never seen, and that is a difficult position to defend.

Next, confirm the surveyor’s marks. The surveyor should provide the control points and the boundary marks in writing, with their coordinates and the datum they are based on. Check that the marks you are using are the ones the surveyor identified, and that the marks have not been disturbed by earlier work on the site. A mark that has been knocked out by the excavator is a common cause of a layout that is wrong from the start, because the crew picks up the nearest peg and assumes it is the right one.

Finally, confirm the datum and the coordinate system the tool is using. A layout that is set to a local grid when the surveyor works in a different system will be offset by a distance that can be large, and it will look correct on the screen. Ask the surveyor which system they used, and make sure the tool is set to the same one before the first point is placed.

The checks that catch most errors before a pour

Once the layout has been placed, check it against the surveyor’s marks by measuring the distance from the boundary to each corner of the building, and comparing that distance with the approved plans. A measurement taken with a steel tape or a laser distance meter, on the ground and independent of the tool, is the simplest test. If the building has a setback from the boundary, check the setback at each corner, not just at one point, because a rotation can move one corner closer to the boundary while the others look fine.

Use the diagonal check. Measure both diagonals of the building, and compare them with the diagonals on the approved plans. If the two diagonals differ, the building is not square, and the difference tells you that a corner has moved. This is the oldest test in the trade, and it is still one of the most reliable, because it catches rotation and distortion that a single distance cannot show.

Check the level as well as the plan position. A slab that is placed in the right position but at the wrong height will cause trouble with the drainage, the floor levels and the threshold to the garage. Confirm the reference level with the surveyor, and check the level of the forms or the pegs against it with a level or a laser. A layout that is right in plan and wrong in level is still a wrong layout.

Check the footing centrelines and the faces separately. Many digital tools show the centreline of a footing, while the form boards and the excavation are set to the face. A mismatch of that kind can put the footing a little off the design position, and the error only becomes visible when the slab edge is checked against the boundary. Confirm which line the tool is showing, and make sure the crew is setting the forms to the same line.

Recording the check so it can be proved later

A setting-out check that is not recorded is very hard to prove to a certifier or a court. Record each check with the date, the revision of the plans used, the surveyor’s reference marks, the measurements taken and the result. Photograph the marks and the measuring equipment in position, and note who carried out the check and who signed it off. The record should be filed in the job file before the concrete is ordered, not reconstructed after the pour.

Keep the coordinates the tool used, the datum and the revision of the model in the same file. If the layout has to be reset because the marks were disturbed, the new reference and the new check should be recorded as well. Digital tools log a great deal of information automatically, and that log is useful evidence. Make sure it is exported and kept, rather than left on a tablet that gets wiped or replaced.

The discipline ties in with the wider logging of site measurements. The article on digital measuring tools that log results covers how to keep the results of a measurement in a form that can be produced later, and the same principle applies to a setting-out record.

What to do when the layout and the reference disagree

Sometimes the check fails. The layout measures differently from the approved plans, or the diagonals do not agree, or the level is out. The first step is to stop. Do not pour the slab, do not move the forms by feel, and do not average the difference out. Re-check the measurements, because a mistake in the check is more common than a mistake in the layout, and then check the reference again. If the reference is confirmed, the layout is wrong and must be reset.

If the reference is in doubt, call the surveyor. A surveyor who is asked to check the marks will often find that a control point has been disturbed or that the wrong mark was used. Do not attempt to re-establish the boundary yourself using the nearest fence line or a neighbour’s mark. The surveyor’s reference is the one that counts, and it is the reference the certifier will accept.

Once the layout is confirmed, record the reset, the new check and the reason for the change. A reset that is recorded properly is an example of good practice. A reset that is quietly made and not written down is the first thing a certifier will ask about when the next inspection arrives.

Digital tools and the safety of the crew

Digital setting out brings its own hazards. The crew works near machinery, often on a site where the ground is uneven and the equipment is set up on tripods or poles. The guidance from Safe Work Australia on digital technologies covers the way that new tools change the risks on a site, and the same thinking applies to a robotic total station or a GNSS rover. Set up the equipment where it cannot be knocked over by the traffic on the site, and keep the crew out of the path of machinery that is working near the instrument.

Make sure the operators are trained on the tool they are using. A digital layout tool that is operated by someone who has used it once is a risk to the accuracy of the job and to the safety of the people around it. Agree who is responsible for the set-up, the check and the sign-off, and make sure the person who checks the layout is not the same person who placed it.

Check the signal and the environment as well. A GNSS rover near tall structures or heavy tree cover can lose the signal, and a layout placed on a degraded signal can be out by more than the crew expects. Note the conditions at the time of the layout in the record, so that a later check can explain why a result is what it is.

Worked example: a double garage and a slab on a tight boundary

Consider a builder in western Sydney who was placing a slab for a single-storey dwelling and a detached double garage on a narrow block. The layout was done on a tablet with an augmented reality overlay, and the crew placed the forms to the tablet’s lines the same morning. The builder had used the tool on many jobs and did not think a second check was needed, and the surveyor’s marks had been placed on the block a week before.

The check was done only because the builder’s supervisor had a habit of measuring the setback before any concrete was ordered. The supervisor measured the distance from the boundary to the garage corner nearest the neighbour, and found that the layout was closer to the boundary than the approved plans showed. The model had been built from a revision that had been superseded by a minor change to the garage position, and the change had not been picked up in the tool.

The builder stopped the pour, brought in the surveyor to confirm the marks, reset the layout from the approved revision and re-checked the diagonals and the setback. The forms were moved before the concrete was ordered. The change cost a morning and a second pair of hands. A setback that was not caught would have meant a garage built in a place the certifier had not approved, and a neighbour with a complaint and a lawyer.

Where this fits in the wider job

Setting out is the first measurement on a job, and every later measurement depends on it. The walls, the frame, the roof and the services are all placed relative to the slab, and an error at the start carries through the whole build. The check against the surveyor’s marks is the cheapest insurance the job will ever buy, and it takes less time than the argument that follows a wrong layout.

Other tools can help with the wider picture. The article on site cameras and time-lapse records covers how a visual record of the site can support the setting-out file, and it sits alongside the setting-out record rather than replacing it. Thermal imaging and other site tools produce evidence too, and the same question applies to all of them: can the output be checked against something independent?

If you are keeping your licence current, the PointCPD guide to NSW builder CPD explains the renewal requirement in plain terms. Digital tools are changing how builders work, and the habit of checking the output against an independent reference is the part of the change that will last.

Questions builders ask

• Digital Tools
• Footings
• Site Setup

Published 13 October 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.