Level of Development in Digital Construction and Reality Capture

[edit] Introduction

Level of Development (LOD) is a framework used to describe the extent to which a Building Information Model (BIM) element has been developed in terms of its graphical representation, associated information and intended uses. It provides project participants with a common way of communicating the extent to which model elements have been developed and the degree to which they can be relied upon for particular purposes.

LOD is particularly associated with the framework developed by the American Institute of Architects (AIA) and subsequently developed through industry guidance including the BIMForum Level of Development Specification. The framework defines a series of levels from LOD 100 to LOD 500. It is not, however, a universal international standard, and definitions and applications of LOD can vary between information-management frameworks and projects.

In reality capture and scan-to-BIM workflows, LOD can be used to specify the required development of model elements derived from point cloud or other survey information. The required level should be determined by the intended use of the model and the project's information requirements rather than by the capabilities of the surveying equipment alone.

[edit] Level of Development and related terminology

Level of Development should be distinguished from Level of Detail. Level of Development describes the extent to which a model element has been developed and the uses for which it can reasonably be relied upon. Level of Detail generally describes the amount of graphical detail represented in a model element. A visually detailed object may therefore have a relatively low Level of Development if its dimensions, location or design have not been sufficiently established.

The distinction is important in reality capture. A point cloud may contain a very high density of measurements and show considerable geometric detail, but this does not automatically mean that the corresponding BIM elements have a high Level of Development. The point cloud is measured survey data, whereas the BIM model is an interpreted representation containing defined objects and information.

LOD should also be distinguished from the level of information associated with a model element. Graphical information describes the geometry and visual representation of an element, while non-graphical information can include specifications, classifications, performance characteristics and asset information. Project information requirements should establish what information is required and when it is to be delivered.

The ISO 19650 series provides an international framework for information management using BIM but does not establish the AIA/BIMForum LOD 100–500 classification as a universal requirement. Projects using ISO 19650 should establish their information requirements, information delivery milestones and methods of defining the required information and geometry.

[edit] LOD stages

The commonly used LOD framework comprises six stages, from LOD 100 to LOD 500. The stages describe increasing development of model elements, although the precise requirements should be established by reference to the applicable LOD specification and project requirements.

LOD 100 represents a conceptual level. Elements may be represented by symbols, masses or other generic representations that communicate their existence or broad spatial characteristics without defining their precise geometry.

LOD 200 represents elements as generic or approximate objects. Their size, shape, location and orientation may be approximate, and the model is generally suitable for applications where detailed or construction-level information is not yet required.

LOD 300 represents elements with specific geometry and information sufficient to describe their size, shape, location, quantity and orientation to a defined level of accuracy. At this stage, elements can provide a more reliable basis for design documentation and coordination.

LOD 350 develops elements to include information about their relationships and interfaces with other building systems. This can include supports, connections, penetrations and other characteristics needed for more detailed construction coordination.

LOD 400 represents elements developed to a level suitable for fabrication, assembly and installation, where this is required by the project. The model may incorporate detailed information concerning individual components, connections and fabrication requirements. LOD 400 should not automatically be assumed to represent the requirements of every fabrication process, as these depend on the relevant trade and project.

LOD 500 is generally associated with model elements that have been verified against the completed or existing asset. It represents a field-verified condition rather than simply a further increase in geometric detail. The information associated with LOD 500 elements should be appropriate to the intended operational or asset-management uses of the model.

The LOD stages should not necessarily be applied uniformly to an entire model. Different elements can be developed to different levels according to their purpose, design status, procurement requirements and information needs.

[edit] Application to reality capture and existing buildings

Reality capture techniques such as terrestrial laser scanning, mobile laser scanning and photogrammetry can provide measured information from which BIM elements are developed. In a scan-to-BIM workflow, point cloud data is normally registered, checked and processed before being used as a reference for modelling.

The quality and accuracy of the source survey do not, by themselves, determine the LOD of the resulting BIM elements. A high-resolution point cloud can support the development of highly accurate geometry, but the model element must still be interpreted, classified and developed in accordance with the requirements of the applicable LOD specification.

Existing buildings present particular challenges because their geometry may differ substantially from idealised design assumptions. Walls, floors and structural elements can be displaced, warped, bowed or otherwise deformed through settlement, movement, alteration and deterioration. Modellers may therefore need to represent irregular existing conditions rather than forcing them into simplified orthogonal geometry.

The limitations of reality capture should also be recorded. Laser scanning generally requires a line of sight between the scanner and the surface being measured, so concealed areas may not be represented. Point clouds cannot normally establish the construction or condition of concealed elements such as internal wall build-ups, reinforcement or foundations. Additional survey, inspection or investigation may therefore be required before such information can be incorporated into a BIM model.

For refurbishment and heritage projects, a model can contain a mixture of measured, inferred and assumed information. These distinctions are important when assessing whether an element can be relied upon for design, construction or asset-management purposes. Model development should not imply a level of certainty that cannot be supported by the available evidence.

[edit] Limitations and project requirements

LOD is a communication and information-management framework rather than a guarantee of geometric accuracy or model quality. A model element assigned a particular LOD does not necessarily have a universally defined positional accuracy. Accuracy requirements should be specified separately where they are critical to the project.

The cost and effort required to develop model elements generally increase as the required geometric and information content becomes more extensive. Developing all elements to the highest possible level can therefore result in unnecessary modelling, larger files and increased processing requirements. The appropriate LOD should instead be determined by the decisions and activities that the model needs to support.

Projects should define how LOD is to be interpreted, which model elements require each level, who is responsible for developing them, and at what information exchanges they are to be delivered. For projects involving existing buildings, requirements may also need to specify survey accuracy, tolerance, point cloud density, coverage and methods for identifying areas where information is incomplete or uncertain.

A clearly defined LOD framework can reduce ambiguity between project participants and help ensure that model information is appropriate for its intended purpose. It should, however, be used alongside the project's wider information requirements, quality assurance procedures and information-management processes rather than as a substitute for them.

[edit] Related articles on Designing Buildings

• Level of Development LOD
• BIM levels of development
• Level of detail for BIM
• BIM dimensions, maturity and levels of development
• How to Select the Appropriate LOD for BIM?
• A Comprehensive Guide to Choose the Right LOD for Your Scan to BIM Projects
• Building Information Modelling BIM
• Scan to BIM: Enhancing Building Information Modeling with 3D Laser Scanning
• Point cloud
• Clash Detection in 3D BIM Models