Accuracy Explained

 

Why is accuracy important for contractors?

Is it true that if you’re precise, then accuracy is not too far away? What if I told you they are entirely different? The two are very much independent of one another. Accuracy is still 100% necessary to your construction process, and the benefits of accurate measurements vs the costs of rework speak for themselves. So, what does it entail exactly, and why do you need to care?

Accuracy vs Precision

To uncover accuracy, we need to understand the terms that make accuracy, their difference with precision and how they’ve come together in construction. 

• Accuracy
  • It is the agreement of an obtained value to a known “true” value. The dartboard, for example, could be represented as how close your darts are to the bullseye. With construction positioning, the obtained value would be the resulting layout from tracing on-site and the known “true” value is the correct positioning. Ideally, there would be no differences between those two. Moreover, there are two types of accuracy construction methods used: relative and absolute.

• Relative Accuracy
  • Relative accuracy is how close a measured value is to a standard value on relative terms, in other words, independent of scale and translation. For example, our laser projector can project a plan by positioning itself within the existing environment (e.g., a room on a floor). It’s using its reference point system to place and position materials and walls from within a space and not a plan.

• Absolute Accuracy
  • Absolute accuracy is how close a measured value is to a known absolute true value. For example, a projected laser line would be perfectly positioned as referenced in the plan. The typical agreeable values are between 3mm to 6mm of the “true” determined value. So, if your laser/chalk line is 3 mm to the right side of the wall line as shown on the plan, it is within range of absolute accuracy. Hence, why a construction tool will be “precise by” rather than precise. Our laser projector can project by positioning itself on a floor with the reference coordinates of this floorplan (x, y, z coordinates on a digital layout).

• Precision
  • To start, precision is independent of accuracy. It represents how multiple close measurements are to each other. Being precise is equal to a set of close values, but not necessarily close to the true value (bullseye). If values were close together and near the “true” value, it would be precise AND accurate. Ideally, the result you should get. Moreover, precise measurements are both repeatable and reproducible.  So precision is all about being constant and getting the same results in an unchanged environment. In robotic total stations, precision will be determined by two measurements: the laser distance measurement (measured by the rangefinder) and angular measurements (arcsecond), which are determined by the angular sensor.

 

• Repeatability
  • One worker operating an instrument gets the same measurement over and over. For example, a laser projector handled to project lines should be displaying the same positioned lines again and again. At Mechasys, we are ensuring our laser projector tool’s repeating ability by setting three types:

Repeatability 1: The projected real-scale plan does not move and keeps its position as our laser projector FramR rotates on itself. 

Repeatability 2:  The projected plan does not move and is projected in the same position every time our laser projector is being turned on/off or has been moved and repositioned.
Repeatability 3: The laser projector is moved on the field but keeps using the same line as the axis to project the plan (plan projected stays in position).

 

• Reproducibility

Multiple operators handling multiple instruments get the same measurement over and over. Again, various tools of the same brand should show reproducible results.

 

Why BIM projects bring on-site accuracy to another level?

The way construction is done hasn’t drastically changed in the last decades. Materials may have, the labour force has evolved with the technique, but fundamentally, construction practices have stayed. So, why is the issue of accuracy coming up today? Accuracy is probably THE factor that will ensure your construction project will run smoothly. Think about it: if your measurements in the construction phase aren’t accurate, there is 100% chance rework will need to be done onsite, resulting in delays in the project. It is more efficient to validate your measurements’ accuracy than to deal with challenges such as a misplaced wall or plumbing sleeve. So why are we noticing this issue just now? In one word: BIM

The utilization of BIM from the beginning in the pre-construction/design phase and throughout the project’s lifecycle brings accuracy to another level. The BIM coordination process consists of aligning elements at a high precision rate on a digital construction layout. Using Computer-Aided Design (CAD) software, the digital construction layout is mm accurate, sometimes even more. So, the core advantage of using BIM in the design phase: minimized rework risk by sharing the model with different contractors involved. This way, communication is facilitated, and design measurements have a better probability of being accurate because there is a combined effort to detect clashes or issues within the model. Then why not do it on-site? You want to be sure that your equipment and labour are performing those same accurate measurements at the construction process. Working in an integrated fashion with technologies like BIM will lead to a more seamless workflow and accurate BIM-to-field execution with the digital construction layout. However, if your positioning isn’t accurate, sadly, there’ll be a price to pay.

Cost of a lack of accuracy: rework

What we’ve seen: one of our clients was installing the flooring materials of a condo building when it was brought to attention that some measurements were wrong about the party walls and corridors. Consequence: the whole thing was to be redone; that means multiple trades/ different companies having to redo their work; framing, plumbing, electrical and mechanical engineering. Of course, everybody was frustrated (although no more than the client, we’re sure). Imagine the cost! Re-ordering materials, having labour come back to redo the job, … and the delay of the project completion. And don’t forget the legal litigations.

By guaranteeing accurate measurement, your projects can be led faster to completion. No rework also means you are saving time, labour force and money. All good things for your company, your reputation, and the relationship with your clients.

Now this question should come to mind: how to ensure accurate positioning? Construction tools such as layout projectors and skilled workers are key to this smooth construction process. That is why it so important for tools to be calibrated (quality test) and for workers to be trained to obtain the specific skillset needed.

The Importance of Calibration

Construction tools such as surveying total stations require precision. The companies producing such tools guarantee their precision and reproducibility. That’s why in the manufacturing phase, they are tested to ensure those two criteria are met through the calibration procedures. That is where the magic happens. And it should be done again throughout the lifecycle of the product. It is important to maintain calibration procedures on the equipment, depending on the manufacturer’s notice. But here’s the missing part: preciseness does not equal accuracy. So, what more is needed for an accurate positioning?

The Importance of Skilled Labor

Accurate positioning will be performed by workers. This is why they should receive the appropriate training and develop the skillset needed. The workers need to possess adequate skills to operate the positioning tools. If the machine is guaranteeing repeatability and reproducibility (preciseness), it’s the person operating it, combined with on-site data, that ensures accuracy and repeatability.

Now you know what accuracy is and why it is so important in the construction process. Precision is all about reproducible results where measures are close, and accuracy is those precise measurements positioned at predetermined coordinates. Technologies like BIM and its digital construction layout are solving accuracy needs in plans and models. For field execution, tools like laser projectors and skilled labour are helping achieve accurate positioning, as designed on the plan. By having a flawless digital layout design, where every trade can validate measurements beforehand, accompanied with precise tools and human skills to achieve accuracy on-site, should we expect an increase in prefabrication? Depending on project factors like size and complexity, it could mean greater ROI, reduced materials costs and labour. It certainly could mean more prefabricated walls and units. What are your expectations for BIM’s accuracy impact on prefabrication?