Dimensional Accuracy – What is Achievable in the Field?

Dimensional Accuracy – What is Achievable in the Field?

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When I am asked to speak about dimensional accuracy, I often refer to a personal story that happened to me when I began my career as a young structural designer. At that time, I was eager to impress the design checker with my ability to accurately dimension structural rebar spacing in a spread footing foundation. I wanted to make sure the rebar was evenly spaced based on design criteria and clearance. Without even considering the practicality of what I was doing, I placed the rebar spacing dimension of #5 rebar at 7 7/16” on center spacing. As I proudly handed over the drawing to the checker, he looked over the work, and a few minutes later he told me that we were going to the field to do some field measurements the next day. The next day, we proceeded to the site, and the checker was looking for some foundations that the concrete had not been poured yet, but the form work and reinforcing steel were in place. When we found a foundation that was ready for pouring, the checker asked me to go verify the rebar spacing to a 1/16“ to make sure it was spaced correctly. Eagerly, I started the process of checking the dimensional accuracy of the rebar spacing with a tape measure. After 30 minutes of taking measurements, it became apparent to me that it was impossible to achieve the 1/16“ accuracy because the rebar had bends in it. It was then that I learned about the practicality of dimensional accuracy as it relates to construction, and the senior design checker never had to say a word. To this day it was one of the most important teaching moments that I have experienced.

Dimensional accuracy is one criterion that is very important in every task that involves manufacturing or constructing something. The challenge is determining what is a practical accuracy, and what tolerances have been determined to achieve the intended result. The intent of this article is to provide a few considerations when the subject of dimensional accuracy is raised. 

Fundamentals Still Apply

If the there is one lesson I try to emphasize to young people, which was instilled into me from my predecessors, it was to remember the fundamentals when you perform a task and never assume anything that is important. Measurement is a very fundamental activity that has been used since mankind started to build and construct. Over time there has been a fundamental rule associated with measurement that still applies today and that is the “Measure Twice and Cut Once” rule. I still use it not only in my professional role, but also in personal projects that require measurement. However, there is a trend that is starting to develop, which involves the use of measurement technologies that automate the measurement process, and as a result, the tendency is to eliminate the need of a second check and assume the technology has made the correct measurement and accept it as absolute. This is a dangerous precedent, due mainly to the fact that all measurements whether done by traditional manual methods or using a sophisticated measurement technology has an inherent measurement uncertainty.

If you look at most construction documents today that are extracted from a 3D CAD model is that everything in the model is dimensionally absolute, and the documents extracted from the model have absolute dimensions not matter what the dimension is. In addition, there are no tolerances factored into the documents because the tools are designed with high precision. As a result, generated construction documents from the 3D CAD model will have dimensional information documented as precise to a 1/16″ , which is impossible to achieve in the field, and is not practical for construction since most construction sites have movement beyond a 1/16″. Furthermore, since tolerances are not factored into these construction documents, which is a necessity in most cases, then there is a risk that is introduced that had not been accounted for. In the days of producing construction documents manually, tolerances of ± 1/4″ were documented, and we never dimensioned anything below a 1/4“ because we understood the practicality of what measurement accuracy could be fundamentally achieved in the field.

Technology is Still a Tool

Measurement technologies like terrestrial laser scanning, mobile and aerial lidar scanning, and high precision robotic total stations (survey instruments) have revolutionized the way we capture measurement information today. These technologies have given us the ability to capture more accurate measurement information more efficiently, more cost-effective, and provides a better-quality deliverable. If these benefits were not enough, they also have a visual component that integrates with design modeling tools, which literally brings the site to the desktop. However, with all these benefits, we still need to recognize that these are technologies, and all technologies are tools. To use these technologies effectively we need to understand what the limitations of the technology are, and what is achievable with the technology. It is also important to note that technology is only as good as the person who uses it.

A measurement technology like terrestrial laser scanning, has been successfully used for collecting measurement information of existing condition, and using this information as a tool to execute capital project work, but the success was not the technology alone. The technology coupled with the correct work process and skilled technicians using the technology to produce an accurate deliverable is what makes it a success.

However, with this repeatable success there are service organizations, manufacturers of this technology and customers that think laser scan technology can be used for everything and expect unrealistic accuracy results because a technology manufacturer claims it is achievable in the field. As a result, assumptions are made, and deliverables fail to deliver the claimed results, and the technology develops a tarnished image.

To avoid falling victim into using the wrong measurement technology, educating yourself on what is achievable and testing the results is the best method to mitigate the risk. You may even need to solicit a subject matter expert (SME) to go over the various measurement technologies available and discuss what is dimensionally achievable for each technology.

Accuracy and Precision Are Not the Same

Over the years, working in the measurement services and measurement technology industry I seem to always hear this one question, “How accurate is the technology?” I always follow that question with the response of “It depends on several criteria.” Few of those criteria are, the distance of the item measured, conditions of the item being measured, the method of how the technology is being used to perform the measurement, and the type of equipment being used to collect the measurement information.

As mentioned earlier, the terrestrial laser scanning technology has become a very popular tool to use in gathering as-is measurement information; unfortunately, there is a lot of mis-information being distributed about the capabilities of what accuracy is achievable. Most information that is distributed or quoted are based on precision test results. The results of the tests are then documented on specification sheets. When this information is distributed, most read the specifications and assume the precision tests are the same as accuracy results, which is a false assumption, and has led to many failed accuracy expectations using the technology. The important thing to remember regarding the difference between accuracy and precision is that accuracy is true to the intention; precision is true to itself. An example of this would be the tape measure, the tape measure is a standard of measurement tool that has measurement marks to the 1/16″, which means that this measurement tool is designed to achieve measurements to a 1/16″. However, if this was a 50-foot measuring tape, and we were trying to measure something that was 50 feet away with a 1/16″ accuracy then it would be very difficult to achieve since distance, movement, and tape sag are factors that are introduced, which will cause uncertainty into the measurement.

The terrestrial laser scan technology device has the same constraint issues as any other measurement device in the field. The specifications may document the measurement accuracies of an individual laser scan in a controlled calibration lab environment, but when the same laser scan system is then taken to the field, measurement results in the laser scan will vary due to the distance of the object being measured, the angle of the laser hits the object being measured, the reflectivity of the object, the movement of the object, the movement of the laser scan device itself, and finally how the scan is registered to the physical coordinate location of the object in the field. All these factors introduce error into the final measurement solution, and if the person is not experienced or knowledgeable in taking this into consideration then significant measurement errors will be the result.

Conclusion

The inclusion of measurement technologies on a project for collecting measurements in the field is very beneficial to everyone on a project and can save significant costs by providing everyone with consistent measurement information, which will save time and mitigate potential measurement errors. However, like any measurement device, consideration to the limitations of the technology and the field conditions must be considered to understand what the achievable accuracy is. Field measurements are never absolute, and tolerances should always be factored into measurement. So, always remember when it comes to field measurements, no matter what technology is used to perform the measurement activity. You should always measure twice and take the average of the results, and then factor in a tolerance.

For more information or to receive a quote:

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or Call Matthew Craig (832) 372-6212

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About The Author

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Matthew Craig has been working in, and servicing the Engineering, Procurement and Construction (EPC) business segment within the oil and gas and chemical industries for the past 38 years. For the past 20 years, he has specialized in dimensional control hardware and software technologies that focused on 3D data capture. His experience includes product planning, product management, project management, software business development, software channel management, systems analyst, application programmer, and structural designer.

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