D laser scanning and CAD modelling are now well established survey techniques, used extensively for the accurate mapping of large scale development projects and complex buildings, as well as smaller projects. Often, however, laser scan surveys are requested when the specified tolerances call for higher level dimensional control techniques, applied using more traditional instrumentation.

Data accuracy

Whilst ‘fit for purpose’ for many applications, laser scanning is subject to inherent limits on accuracy, with terminal accuracy dependent on a number of variables.

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 2-10mm, 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 +/- 2mm should be approached with caution. Using the highest possible specification terrestrial laser scanner in a stable, controlled environment at relatively short range (10-20m) and with high order survey control (+/- 1mm) 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 +/- 5mm being more realistic.

Moreover, the concept that modelled surfaces are more accurate than the basic cloud data is only true in certain circumstances. Most construction related software modelling packages were originally intended to represent design rather than the less predictable “as built” environment. Where projects need to address the interface between existing structures and new design, there may be inconsistencies that need to be acknowledged and addressed in a different way.

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. Although resulting in a very good looking model, this may not accurately represent what actually exists on site. In this instance, the accuracy of the model is dictated by the quality of the structure being surveyed. While a number of software companies are currently investigating how such limitations can be overcome, a commercially viable solution is still some way off.

An alternative option available to data users is to bypass the automated software corrections 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 to address purely dimensional issues.

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. Often however there is no substitute for a full CAD model.

Laser scanning or dimensional control? So how do you decide what level of input 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 relates to high accuracy techniques and instrumentation used to achieve first time ‘fit up’ on site between new and old complex pieces of plant or structures.

Ultimately, designers and construction professionals need to be clear about what the project demands. Equally, 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.