BIM and building surveying: laser scanning
25 September 2017
Shelby Green explains how laser scanning can be used with building information modelling in surveying buildings
Laser scanning is becoming the preferred method for recording as-built 3D data quickly and accurately. This data can be processed and used for a variety of purposes, appealing to many professions in construction.
A point cloud generated by laser scanning is effectively an accurate 3D representation of a real object or environment. Time-of-flight scanners measure distances by firing a laser beam at a surface and calculating how long it takes to return.
Laser scanners can record up to a million of these points per second, each with an x, y and z coordinate. As scanners work using a line of sight, the scan will only record what you can see, so in order to gain full coverage of a building, multiple scan set-ups need to be taken at various locations.
Each scan can take between 1 minute and half an hour, depending on internal accuracy settings. The individual scans are then collated using Leica Cyclone software, which allows for automatic overlap recognition and target-based data registration. This does mean that each scan must share details with the next one to allow for point cloud overlapping, however.
Before the scanning takes place, a survey control network must be installed using robotic total stations to ensure accuracy. These calculate rounds of angles using prisms placed on each visible control point to establish known locations. Externally, control points usually comprise survey nails driven into hardstanding, whereas for internal control temporary self-adhesive targets are attached to walls and floors.
Figure 1: BIM image of the library at RICS’ headquarters
Once a survey grid is established, scan targets are placed on each of the known locations and recorded by the scanner to transform the point cloud on to the desired survey grid; this is called a controlled scan. Uncontrolled free-station scans, which do not have an established location, are taken between control points and later aligned to controlled scans that share overlapping detail.
Controlled scans essentially act in the same way as the corner pieces of a jigsaw, which is why it is important to have a robust survey control network installed before you carry out a laser scan survey.
While this methodology is used for static scanning, other methods such as mobile scanning – which is done while moving with a hand-held scanner or one on a trolley – are emerging for internal scanning. Currently, the accuracy and quality of mobile scanning is questionable; however, it does offer a much quicker and more efficient workflow.
Why laser scanning?
With advances in technology, laser scanners have become quicker, lighter and more accurate. Therefore the process has become far more efficient and financially viable for businesses. Scanning also offers huge benefits compared with more traditional methods.
The main reason why there has been increased interest in laser scanning and building information modelling (BIM) in recent years is the government mandate requiring all centrally procured projects to be completed to BIM Level 2 from 2016.
The point cloud is a measured survey represented as billions of points and is itself fairly unintelligent. For the purpose of creating a building information model for an existing asset, the cloud is simply a way of producing accurate 3D solid geometry with attributed information.
However, laser scanning is not just limited to BIM. Severn Partnership for instance has many clients that require the point cloud as the main product, for purposes such as archiving assets of archaeological or historical interest, monitoring movement, detecting defects, recording deterioration of materials over time and structural analysis.
Some previous projects that have used mobile scanning to monitor defects have included rock formations and furnaces for glass manufacturing. Scan data has also recently been used to establish the extent of structural movement in historic buildings and retaining walls.
Building information models can be used for a variety of purposes throughout a building's lifecycle. Although many assume that BIM is predominately used for design and construction, this only contributes to a very small proportion of a building’s lifecycle and cost.
BIM is about using data to manage the asset effectively. The data can be further used for clash detection, 4D scheduling – which represents the project programme in a digital form – and 5D quantity take-off. Some professions that are fully implementing BIM include owner-operators, asset managers, architects, contractors, engineers and heritage specialists.
Figure 2: A 3D model of RICS’ Parliament Square headquarters
Almost any necessary information can be included in a building information model depending on the proposed uses, such as installation dates, materials and sustainability parameters; for building surveyors, this may include planned maintenance schedules. By applying this information, the surveyor ensures that all appropriate building elements are included in the schedule, as well as monitored and updated.
While information enables better understanding of how an asset is managed, 3D geometry allows full visibility of the way the building is constructed. This can lead to accurate defect analysis and therefore reduce the number of assumptions made. If there is no existing building information model, laser scanning alone is an efficient and accurate way to establish structural movement. Scans can be made in true colour, which may further emphasise any defects present.
Costing information is often attributed to components in building information models as well, to allow estimations to be made throughout a project. Costing information should also be re-used for other purposes such as performing reinstatement cost assessments to offer a much more efficient and accurate outcome than the current process does.
Currently, the most common use for the Scan2BIM process, in which a point cloud is used to generate a model, is refurbishment projects. Using laser scan data to develop 3D models allows architects to create accurate designs. These models should then be used by building surveyors as well as other stakeholders involved to increase collaboration, ensure all data is fully coordinated and contextualise the specification process.
Having all graphical and non-graphical information in one database eliminates the need for incomplete or out-of-date paper-based operation and maintenance manuals. It is the ability to access this information efficiently that will have an impact on day-to-day tasks such as pre-acquisition surveys.
For instance, I was recently approached by a building surveyor who was interested in commissioning a laser scan survey of a number of residential and retail units to appraise the service charge recovery clauses of each commercial lease.
Arguably, laser scanning may seem unnecessary in this case when traditional methods could be used. However, the client was also interested in elevations, sections and 3D modelling for other purposes. Along with advancements in technology, this is when laser scanning becomes far more viable.
Other specialists such as measured building and right to light surveyors are using the technology to its full potential. New rules are continually governing how measured data is recorded and used in the building industry. A recent example is the International Property Measurement Standards: Residential Buildings, which establishes a consistent methodology for measuring such buildings around the world.
For the right to light surveyor, 3D models can be used to make an accurate analysis and extract the necessary sunlight calculations. Models may also be used as a reference to support dispute resolutions in relation to Building Regulations and other standards.
It is important that surveyors and major stakeholders understand the merits of laser scanning and BIM. If an existing building information model is available, building surveyors are more likely to use the data for the variety of work they have.
Widespread BIM implementation is leading to new efficient workflows, affecting every profession in construction.
Shelby Green is Scan2BIM Manager at the Severn Partnership