Existing Conditions Survey in Digital Construction
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[edit] Introduction
An existing conditions survey is the process of measuring, inspecting and documenting the physical characteristics and condition of an existing building, structure or site at a particular point in time. Depending on the requirements of the project, it can record dimensions, geometry, materials, visible building elements, services, defects and other relevant information. The resulting information can provide a reliable basis for architectural design, structural engineering, construction planning, refurbishment, maintenance and facilities management.
Existing conditions surveys can produce conventional drawings and schedules, photographs, survey data, point clouds or three-dimensional digital models. The extent and accuracy of the survey should be defined according to the intended use of the information rather than assuming that the most detailed survey method is always required.
[edit] When existing conditions surveys are required
Existing conditions surveys are particularly important for refurbishment, alteration, extension, adaptive reuse and conservation projects where the physical building may differ from available drawings and other historical information. Original drawings may not reflect subsequent alterations, construction tolerances, deterioration, settlement, movement or the installation of building services. Previous work may also have been poorly documented or undocumented.
A survey can establish the geometry and visible characteristics of the existing building and identify discrepancies between available documentation and physical conditions. Depending on its scope, it may also identify structural movement, defects, alterations and building services that could affect the proposed work. Surveys may therefore help design teams identify constraints and coordinate proposed work before construction begins.
An existing conditions survey is not necessarily the same as a condition survey. A measured or spatial survey primarily records the physical geometry and location of existing features, whereas a condition survey assesses the condition of those features and identifies defects or deterioration. Both may be required on a refurbishment or alteration project.
[edit] Survey methods and technologies
The method used should reflect the size and complexity of the asset, the required accuracy, accessibility, available information and intended use of the survey. Common methods include manual measurement, total station surveying, laser scanning and photogrammetry. These methods may be used individually or in combination.
Manual measurement can use equipment such as measuring tapes, laser distance meters, levels and plumb lines. It can be appropriate for relatively small or simple surveys, but accuracy depends on the equipment, survey procedure and competence of the surveyor. Manual measurement may be less efficient for complex geometry or large numbers of individual measurements.
Total stations use electronic angle and distance measurement to establish the coordinates of selected points. They can provide accurate survey control and discrete measurements and may be particularly useful where specific points, levels, boundaries or structural elements need to be established. Unlike laser scanning, however, a total station does not normally capture a continuous dense representation of all visible surfaces.
Laser scanning captures large numbers of measurements from surfaces to produce a point cloud. Terrestrial laser scanners can rapidly record complex geometry and can be useful for existing buildings with irregular forms, congested areas or difficult-to-measure features. The accuracy achieved depends on factors including the equipment, range, environmental conditions, survey control, registration and processing. Point clouds are geometric datasets and do not inherently contain information identifying what each point represents.
Photogrammetry uses photographs to derive measurements and three-dimensional information from overlapping images. It can be used for buildings, structures and sites and may complement other survey techniques. Its accuracy depends on factors including image quality, camera geometry, control points and processing methods.
[edit] Digital processing and survey deliverables
Survey data normally requires processing, checking and registration before it is suitable for use. Point-cloud data may be cleaned to remove unwanted or erroneous points and individual scans may be registered into a common coordinate system. Survey control can be established to provide a consistent spatial reference, where required.
The resulting information can be supplied in a range of formats, including two-dimensional drawings, photographs, schedules, survey point data, registered point clouds and three-dimensional models. A point cloud can be used as a reference for creating a three-dimensional computer-aided design (CAD) or building information modelling (BIM) model, but the model is not automatically generated simply by capturing a point cloud.
Where a BIM model is required, modelling teams can use the survey data to construct digital representations of existing building elements. The scope and accuracy of the model should be defined in advance, including which elements are to be modelled, the geometric accuracy required and what non-geometric information is to be included. The resulting model should be checked against the survey data to establish whether it meets the specified requirements.
Typical quality assurance activities can include checking survey control, registration accuracy, dimensional measurements, completeness of the captured area and the correspondence between a digital model and the survey data. The required tolerances should be established before the survey is undertaken.
[edit] Standards, accuracy and information requirements
Survey accuracy should be specified according to the intended purpose of the information. Factors such as instrument accuracy, survey control, measurement range, environmental conditions, registration procedures and the characteristics of the surfaces being surveyed can affect the final result. A statement that a survey is simply 'millimetre accurate' is therefore insufficient without defining the measurement conditions, tolerance and scope.
Information requirements for digital construction projects can be established as part of the project's information management arrangements. ISO 19650 provides a framework for managing information using BIM across the lifecycle of built assets. BIM model requirements can also specify the required geometry, information and level of development of model elements.
Survey specifications should distinguish between the accuracy of the original survey data and the accuracy or level of development of a subsequent digital model. A highly detailed model does not necessarily provide greater accuracy than the survey data from which it was produced.
[edit] Consequences of inadequate existing conditions information
Incomplete or inaccurate information about existing conditions can introduce risks during design and construction. For example, discrepancies between drawings and the physical building may result in poorly coordinated designs, unexpected site conditions, changes to construction details, rework or delays. These risks can be particularly significant where prefabricated or tightly coordinated components are being installed in an existing building.
The appropriate response is not necessarily to use laser scanning for every project. The survey method and level of detail should be proportionate to the project's requirements and risks. A combination of existing documentation, site inspection, manual measurement, conventional surveying, laser scanning and other techniques may provide the most appropriate basis for design and construction.
[edit] Related articles on Designing Buildings
BIM Directory
[edit] Building Information Modelling (BIM)
[edit] Information Requirements
Employer's Information Requirements (EIR)
Organisational Information Requirements (OIR)
Asset Information Requirements (AIR)
[edit] Information Models
Project Information Model (PIM)
[edit] Collaborative Practices
Industry Foundation Classes (IFC)