Peter Field reflects on what constitutes true dimensional control and above all, the importance of ensuring survey results are ‘fit for purpose’
The development of laser scanning technology heralded a step change for surveyors and construction and design professionals. Where complex geometries are involved or a precision fit is required, dimensional control, which also involves traditional survey techniques, is often more appropriate. Peter Field reflects on what constitutes true dimensional control and above all, the importance of ensuring survey results are ‘fit for purpose’.
3D laser scanning and CAD modelling are well established survey techniques, which today are used across all types and sizes of project. As a long-established survey company operating internationally, Warner Surveys was an early adopter of laser-scanning, and has since pioneered many bespoke applications and methodologies using the technology, which continue to result in new applications and the resolution of previously insurmountable challenges.
A huge leap forward for surveying, laser scanning enabled the remote capture and dissemination of large amounts of data, where previously each data point had to be measured individually and frequently, physically. The technology is now used extensively for the accurate mapping of large scale development projects and complex buildings, including for example heavily congested or inaccessible areas with complicated assets, as well as potentially hostile environments such as refineries and industrial plants.
Being able to call on a comprehensive database offers benefits for collaborative working, where many different influencers require input into the design process. The interoperability of modern 3D models created using a variety of software packages is a major contributor to the BIM workflow approach which is increasingly seen as desirable within the construction industry and now a prerequisite for Government contracts. Drawing together a range of disciplines through collaborative work-sharing, these latest technologies enable performance to be optimised at every stage from delivery of design concepts to on-site construction.
Also key to the Government’s requirements in respect of BIM is access to accurate legacy data for buildings and structures. The availability of a comprehensive database which goes beyond original design specifications to provide ‘as built’ information is vital during the building lifecycle. Assigning intelligence to data provides a tool for asset management and a valuable guide for future extensions and alterations, particularly in clash prevention, where pipes and new elements of plant must be integrated within already congested sites or structures.
Laser scanning or dimensional control?
Laser scan surveys are often requested when the required tolerances actually necessitate dimensional control techniques with more traditional instrumentation. So how do you decide what you need? The following may help:
* Laser scanning is a rapid and reliable method for surveying often inaccessible, complex or congested areas.
* Survey control is the essential, traditional survey activity providing the auditable accuracy to so many survey operations including laser scanning.
* Dimensional Control has become the name given to high accuracy survey techniques used to achieve a good fit up between new, basic pieces of plant.
* Critical Interface Surveying raises the bar for dimensional control and relates to high accuracy techniques and instrumentation used to achieve first time fit up’s between new and old complex pieces of plant or structures.
While ‘fit for purpose’ for many design and construction applications, laser scanning is subject to inherent limits on accuracy, with terminal accuracy subject to and dependent on a number of variables. Good quality survey control, range, reflectivity of the surface, type of scanner, temperature variations, stability of the survey subject and even the symmetry of a survey subject can all have an impact on results.
In the real world, most survey subjects are exposed to a significant temperature change which alone can account for several millimetres’ difference in the data registered. Additionally, the ‘noise’ produced within a point cloud from a flat surface can range from as little as 2 millimetres to 10 millimetres or more depending on the quality of scanning instrumentation and the surface being scanned. Interpolation of this ‘noise’ can lead to further distortion in the data. Where the objects being scanned are vibrating or subject to other influences that can cause movement this will also affect the results.
For this reason, claims that laser scanning can provide accuracy to within ±two millimetres should be approached with caution. Using the highest possible specification terrestrial laser scanner in a stable, controlled environment at relatively short range (ten to twenty metres) and with high order survey control (±1millimetre) it may be possible to produce data to that kind of auditable accuracy. If the data is not auditable then accuracy claims should be dramatically downgraded, with ± 5 millimetres being more realistic.
Moreover, the concept that modelled surfaces are more accurate than the basic cloud data is only true in certain circumstances. Surfaces are often irregular, the geometry of old basic primitive shapes become corrupted and even the lack of perpendicularity between (for example) walls and floors can result in some software squaring off or smoothing out such irregularities which although result in a very good looking model do not accurately represent what actually exists on site; so on this occasion the accuracy of the model can actually be dictated by the 'quality' of the structure being surveyed.
While a number of software companies are actively involved in investigating how these limitations can be overcome, a commercially viable solution is still some way off.
Another option available to data users is to bypass the automated software corrections and more complex ‘as built’ geometries by interrogating the underlying databases directly.
Frequently overlooked in the delivery of 3D models, the point cloud database captured by the laser scanning process offers a valuable resource in itself for designers and contractors. Containing high quality, auditable data which can be interpreted factually, these databases offer the flexibility to interrogate and view point clouds to produce 2D plans and elevations directly. This allows the freedom to request additional deliverables from the original survey team in the form of sections or drill down into specific areas at a minimal extra cost.
In addition, opening up wider access to raw data within project teams can facilitate greater interoperability, although this may be hampered by the availability of the necessary software in some cases. Often however there is no substitute for a full CAD model.
Dimensional control – raising the bar
A more inclusive and wide-ranging approach, the critical interface surveying provided by companies like Warner Surveys calls for a different order of complexity and accuracy in dimensional control. For example, ensuring complex cone to cone connections between new and old plant are achieved on a first time lift and fit or high value reactor/regenerator heads and internal components are replaced in a single lift. The same complex geometry setting out experience has been successfully applied by Warner Surveys to some of the latest generation architectural structures including City Hall, the British Museum Great Court Roof and most recently, the new Tate Modern.
In the quest for greater consistency and faster construction, prefabrication where assemblies that are manufactured under factory conditions and then transported to construction sites for incorporation into building and civil engineering works is also increasingly becoming a feature of many new construction projects from housing and hotels to complex and architecturally demanding designs and infrastructures.
Offering greater programme certainty and lower labour costs, this approach also results in less waste. However, the in situ work abutting prefabricated assemblies requires absolute accuracy to avoid civils interface problems.
As a total solution provider, Warner Surveys applies high order total station survey control, combining laser scanning with proven methodologies such as conventional survey control, dimensional control and critical interface fit up’s and is active in the further development of traditional survey techniques to include drone-based/UAV and vehicle-mounted scanners. It is the combination of Laser Scanning, conventional survey control, dimensional control and Critical Interface surveying that can provide the overall data base for design, BIM and clash prevention applications along with 1mm accuracy dimensional control techniques for specific 'corridors of interest' or tie in points. Warner Surveys was awarded an engineering project for Tengizchevroil Future Growth Project-Wellhead Pressure Management Project with survey teams based in Kazakhstan, Korea and Italy over a four year period.
Increasingly, Warner Surveys is working with project teams and organisations on major long-term projects across a range of areas from construction site engineering surveys to multiple site and large site topographic surveys as well as international oil, gas and energy projects.
Working within a framework agreement or on a call-off basis ensures the availability of support and expertise at every interface to deliver a seamless progression and offers an economic solution, fixing costs through the establishment of an agreed schedule of rates.
In addition, Warner Surveys has established a Quick Quote mechanism to simplify process management and communications by providing a fast turn-round for urgent and additional survey requirements.
Warner Surveys was also one of the first companies to set up an autonomous CAD department, a move designed to give designers and contractors increased flexibility and improved integration, with the reassurance of independent results.
Horses for courses
Land surveyors have a duty of care to ensure the services provided are ‘fit for purpose’ and delivered to the required accuracy. Equally however, designers and construction professionals need to be clear about what the project demands. Ultimately, good survey control techniques must lie behind and support the laser scan, however high its quality, providing auditable proof that there is no degradation in the laser scan due to other outside influences.
Future growth project and wellhead pressure management project
Warner Surveys is now mobilising for the Future Growth Project and Wellhead Pressure Management Project (FGP-WPMP).
Warner Surveys was appointed in 2013 as part of the team providing front-end engineering design (FEED) and engineering, procurement and construction management activities. The FEED stage is now complete and detailed engineering underway.
Warner Surveys has been appointed to oversee all technical aspects throughout with the aim of ensuring first time fit on site –a process now referred to as Single Weld Hook Up (SWHU).
While laser scanning models are being used to ensure clash prevention, process plant design and asset management, the technology will only be deployed during engineering and construction to provide final clearance checks prior to shipping through dimensionally restricted inland waterway systems.
The extent of the project and the off-site fabrication of modules require the application of high-end dimensional control techniques throughout to ensure final correct positioning as per design, working within very tight tolerances of less than +/- 2 millimetres. Warner Surveys will be involved in the full life cycle of every module from fabrication and transportation to on-site positioning and the final hook up.
Working closely with the local fabrication teams from its local office in Kazakhstan, Warner Surveys is responsible for the establishment of Standard Operating Procedures and validating the accuracy of the fabrication process through to sign-off. Constant monitoring and reporting allows any occurrences outside of accepted tolerances to be checked back with designs and the necessary amendments made before shipping to site. As part of this brief Warner Surveys are involved with marking the cut lines for pipe connections. On-site the company will continue to manage the civils interface by setting out demarcation lines and match marks to allow cranes and other module transportation systems to manoeuvre and lower modules into place on the concrete foundations to achieve the delicate fit required. The result is a perfect Single Weld Hook Up, minimising construction timescales and avoiding costly on-site reworking.