Conventional engineering surveying results in the creation of asbuilt drawings at the completion of works on a project. These demonstrate to the project manager and/or client that structures and components have been built to the required tolerances. However, they don’t usually provide a 3D model of the completed works, or even enough data to create one.
A laser scan of the finished product, however, produces a point cloud which thoroughly defines the extent of the works, which can then be modelled into an interactive digital record with exact layout, dimensions and attributes of building components. The process of “Scan to BIM” is completed as follows:
1. Set up control networks to orientate scan. In order to give spatial data real world coordinates, target points must be created by means of surveying with a known Easting, Northing and reference level (RL). The same goes for UAV photogrammetric surveys in the form of Ground Control Points (GCPs). These are subsequently identified in the registration process.
2. Acquire scan data and imagery. Using appropriate equipment for the accuracy and range requirements of the job, perform a scan from as many setups as necessary to acquire an adequate representation of the area. This may need to be done at several stages throughout construction to capture data from structures or services which may subsequently be inaccessible.
3. Point cloud registration. This step consists of fixing common features between scans from each setup point, and processing the data to splice together, or “register”, the scans. With the latest scanning instruments and software, this step has increasingly become an automatic part of the process, leading to “targetless registration”. This saves both field and office time, minimising the need for manual target placement and dramatically reducing manual involvement in post-processing.
4. Data optimisation. Scans often generate “noise”, outlying points and other unnecessary data, for example vegetation (when the ground height is required) or vehicles parked on site. This can be filtered out to obtain a more useful and accurate dataset.
There will now be a large and comprehensive point cloud which accurately represents the 3D position and colours of the physical space. This product is very useful for the creation of elevation views, floor plans and sections, but it is still “dumb” data without any attributes or associated information.
5. BIM Modelling. The final step is processing the 3D point cloud into an object-based 3D parametric model that holds both spatial information and attributes of structural and building components. This is still often done manually by drafters and 3D modelers, using programs such as Autodesk Revit. However, the rapidly evolving BIM industry has now seen advances in the automation of this step as well, as algorithms identify and categorise objects from clusters of points. This is carried out to a varying degree of “fidelity”; the detail provided by the model, usually a requirement of the contract expressed in terms of LOD (Level of Development).
6. Deliverables. The BIM model is handed over in a common format (usually IFC) to enable the client and other users to access and import it. An international standard, ISO 19650, has recently been developed to standardise the presentation and format of BIM information.
Another important end use of BIM data is for integration into GIS programs for applications as diverse as city planning, defence and large scale energy use analysis. Consequently, leading GIS software manufacturer ESRI have developed program extensions for the import of IFC data into their products.
As a project deliverable, a BIM model is increasingly being written into the contract by clients including the state and federal governments in Australia. It is a more efficient means of collecting and presenting asbuilt data, resulting in a much better-quality and more useful record. Contractors and their surveying subcontractors must therefore keep up with the advances in technology if they want to be competitive and tender on future projects.
