Control Tolerance: Managing the Range of Deviation

As designs begin to translate into physical structures, field engineers are responsible for maintaining quality and managing risk across a project’s trades and phases. This requires an ability to manage control tolerance, which is the acceptable range of deviation from design specifications that still ensures all elements will align and won’t lead to compromised structural integrity or costly rework.

An effective approach to control tolerances starts at the beginning of a project, carries through the finishing installation and requires a command of skills that include the technical, the logistical, the managerial and the collaborative. Without those, the smallest of issues, oversights or deviations can translate into compounding issues in the future.

This article is a collaboration between two field engineers, one with a few years of experience and one with well over a decade. It aims to share insights and strategies for mastering control tolerance that are valuable to field engineers who work within construction teams with different experiences and enhance construction practices everywhere. 

Understanding Control Tolerance

Control points and tolerances are referenced by numerous stakeholders throughout the project, including architects, project managers, superintendents and subcontractors. Each trade uses the control points and tolerances as guidelines to ensure their work aligns with the overall design and meets all required standards. Field engineers often aim for accuracy within 1/16 of an inch to minimize the potential for compounding errors, but different trades require different tolerances. Structural trades, like steelwork or concrete, might have broader tolerances because of the complexities that come with handling larger materials, while finishing trades, such as carpentry or glazing, often require tighter tolerances to achieve precise alignments, aesthetics and functionalities. Field engineers are usually responsible for continuously monitoring and evaluating tolerances as the project progresses, as well as keeping all stakeholders informed about adjustments or updates.

Mastering Control Tolerance

Certain practices and mindsets can help improve the ability to effectively establish control points and manage control tolerances. 

Be precise from the outset.

Control points need to be as accurate as possible, as they’re what tolerances will be measured against. Increase precision by using double-checking methods, such as redundant measurements or backsighting, as well as advanced surveying technology. Even the act of marking the control point on the ground can result in a slight deviation, which is why experienced field engineers immediately and repeatedly verify their initial measurements.

Watch for compounding errors.

Small initial errors mean all proceeding work will be done with incorrect information. Even slight deviations can compound as work progresses and site conditions change. Regularly verify control points and tolerances to catch errors early and make adjustments before they compound.

Keep documents up to date.

Compounded errors can be minimized by keeping computer-aided design (CAD) files and as-built drawings as up-to-date as possible. This way, stakeholders can base their decisions on the latest information. Establishing effective workflows or adopting project management software can improve document management and overall collaboration.

Adapt to site conditions.

Balance precision with practicality. Adjust control points and lines, if the original plans didn’t account for the structures and conditions found in the field. For example, maintaining visibility with control points is crucial for most trades. If it becomes apparent conditions will obstruct visibility, a control point can be moved to a better location and documents can be updated accordingly.

Continuously communicate.

Establish effective systems of communication to keep all stakeholders informed of changes and priorities. Regular updates and communication keep the project aligned with its specifications, especially as conditions change. 

Understand trade-specific needs.

Different trades have different tolerances based on the work they do, the materials they use and the project phase they’re working in. To provide accurate guidance and support, become familiar with the different tolerances, refer to contract specifications and confer with trades to understand their needs.

Plan for control point longevity.

Set a point that will be visible and useful throughout the project. Consider potential disturbances such as heavy machinery that might damage or slightly move a control point, material storage that might cover it or the installation of structural elements that could obscure its visibility. If necessary, move the control point and update the documents to reflect the change. Consideration should also be made as to what materials the control point(s) is/are attached to, as changes in temperatures can negatively affect control of the precise point locations.

Technology for Mastering Control Tolerance

Modern tools have enhanced the precision, flexibility and efficiency of managing control tolerance. These technologies improve accuracy and facilitate better communication and collaboration.

• Total Station

  The total station includes advanced surveying equipment that combines electronic distance measurement and angle measurement to establish precise control points. Each station gathers comprehensive data and can be set up at various locations around the site to shoot measurements from multiple angles and positions.
  
  However, consistency of the data requires careful attention and documentation. Because the height of the prism pole is adjustable – which is helpful when sight lines are obstructed – every adjustment needs to be noted to avoid errors in elevation data.

• Automatic Level

  An auto level is an optical instrument used to set elevation. While an engineer can shoot elevation with a total station, an auto level typically provides a tighter tolerance due to its smaller margin of error.
  
  Using an auto level offers several other key benefits: it is smaller, quicker to set up, and it does not require batteries. By keeping elevation control separate from the total station, the risk of errors due to pole height adjustments is minimized. 
  
  The most accurate results are achieved by using the total station for horizontal positioning and the auto level to shoot elevation.

• CAD Software

  This software is used to create detailed control drawings that can be shared with stakeholders for accurate and up-to-date reference.

• Laser Scanners

  These instruments used to capture detailed 3D measurements of construction sites, which allow accurate documentation and analysis of site conditions.

• Building Information Modeling (BIM)

  BIM software can create virtual models of structures that can be used for project visualization, planning and ensuring all stakeholders have accurate and up-to-date control data.

• Data Collectors

  Data collection handheld devices are used with total stations or GPS to record and manage survey data efficiently on-site.

• Drones

  Drones can be used for aerial surveys and inspections to provide a comprehensive view of the site, establish control points and monitor progress. 

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The Power of Experience and Guidance

Even with precise technical details and advanced tools, there’s no substitute for experience and a mindset of continuous learning. One of the greatest challenges of managing control tolerances is accounting for and anticipating all the variables that can cause even the slightest deviation and lead to compounded errors. Over time, those variables become easier to recognize and more intuitive to adapt for. 

Field engineers can actively seek opportunities for formal training and become familiar with innovations, but mentorship is perhaps the most effective tool for quickly and effectively gaining an understanding of control tolerances. Experienced engineers can offer less experienced professionals practical advice and in-the-field guidance based on ideas and practices they’ve learned to be effective. Not only is this in the spirit of collaboration — it also improves the overall quality and success of construction projects. 

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