• Virve Juhola

Point cloud scanning is an efficient method to measure and monitor construction quality

Dedicated consultants are used for control measurements at the new Trimble Finland Offices construction site in Espoo. The consultant for the equipment used is Trimble’s reseller Geotrim, and the measurements are provided by Henry Grönberg of Surgeon Oy.

For example TSC7, the latest in field technology by Trimble, a controller that combines the power of a tablet, laptop and survey-rugged design into one device is used on the NCC site in Espoo, as well as SX10, a scanning total station that captures both high-accuracy total station measurements and true high-speed 3D scans together in a single instrument providing a record-high level of accuracy and measurement performance.

“In an ideal situation, all site measurements are taken using a total station, and BIM is increasingly utilized on the construction site,” says Training and Support Manager Kari Immonen of Geotrim. “The IFC files from the modeling software can be used as source data for the latest measuring software, and the data produced by them is easy to transfer back into modeling in the same format.”

Prefabrication requires precise post-production quality control regarding structures and forms

Immonen recalls that total stations were introduced to both construction and surveying in Finland in late 1980s. Laser scanning began in early 2000, particularly to source data for repair construction.

“After that laser scanning was used outside Finland to follow the progress of construction. BIM-based design and construction require precise point-cloud-type analysis of building components. Compared to traditional methods, obtaining as-built survey data is significantly faster by total station scanning. The point cloud reveals the position and shape of the component – whether the beams are bent, the walls direct and the precast units precisely cast and installed. In Finland scanning for this purpose is rare, but it is much more common in North America where prefabrication is rarer.”

“Now that module-type prefabrication is becoming more common, we need more precise quality control regarding the structures and their forms,” Immonen says. “When I entered the industry, I got to use spirit levels and string lines, but in that time everything was cast in place and there was no need for millimeter-precise steel frames, for example. A wall may have been up to one meter off compared to street alignment, but these days houses are precisely squared and angled, and design and surveying technology helps ensure cost-effective accuracy.”

Use of latest methods requires a plunge in the 3D world

Why is laser scanning not more common in Finland?

“Perhaps it is thought to be a bit intimidating as a method because it involves such massive amounts of data,” Immonen says. “The capacity of an ordinary computer may not suffice, and processes to manage the data are not known. However, the latest computer models offer plenty of capacity, and the amount of data is no longer a problem. You just have to get to know them. People just need to be introduced to the devices and user interfaces. Connecting the device frame with the software is easy. However, the point cloud method does require some knowledge of how to use 3D information.”

“In a 3D model, the design is always available and up to date. The controller and the design software are synchronized at regular intervals. Analyzing the laser scanning results in a reasonable time requires some extra properties from the computer, but once this becomes a routine, it is fast and straightforward. For example, this method of measuring allows checking column verticality, position and deflection and floor flatness and slopes. It is not efficient to do all this by traditional measuring."

Laser scanning is extremely fast compared to traditional methods

According to Immonen, measuring one point takes 1 to 5 seconds using a tacheometer, but using a laser scanner, you can scan 26,600 points per second. The difference shows how much more efficient it is to measure by laser scanning.

“Once the point cloud has been scanned, all data is there, and the image functions as documentation of all structures and details including the nuts and bolts. Once the data has been gathered, you always know what lies underneath which structure. Without this information, MEP engineering may need to be re-done at the very end of the project, which will obviously ruin the entire schedule.”

“In warm climate, it is necessary to measure the thermal expansion of a steel frame, for example, because the difference between morning and afternoon can be significant and affect the installation of the building components. The surveyor needs to take into account where the components will be installed and with which structures they may clash.”

Enriched building information by point cloud scanning and modelling

Immonen recounts client experience:

“Before they needed to filter out extra information from the CAD files, and it took up to two hours to refine a design into a surveyable form. Preparing a BIM-based design only takes 5 to 10 minutes if it is already synchronized. Using this new way of working, building information is significantly richer than before and the the content is always ready to use. I believe the current formats, such as IFC, will be available in one way or another in the future, too.”

“Trimble’s SX10 is a true combination device, and the patented Vision technology is one of Trimble’s definite strengths,” Immonen notes. “Thanks to the integrated camera, the photograph is correctly positioned, and transferred from the device, it can be utilized for further processing and measuring. Nobody else can offer this combination of functionality. From a reseller and trainer point of view, it is a great benefit that all the components work seamlessly together: the device frame, the ruggedized computer and the software. There is no need to think about interfacing because everything has already been configured.”