• Improved information content through aggregated and verified basics. This reduces the degree of uncertainty and increases project stability in terms of costs, deadlines and quality for all parties involved in the construction. By eliminating media discontinuities, information once gained remains available.
• More transparency for all those involved in the project thanks to a central basic model (can also be used as a communication medium).
• Improved planning of the planning by definition of goal, employment and purpose of the BIM methodology in the project.
• Synergies can be used already at the beginning of the model creation during the review of all fundamentals. With the creation of the model of minimum requirements (Retro BIM), it is recommended to simultaneously create a specific technical model (e.g. maintenance, operation).
  This early creation of a specialist model for operation and maintenance, enriched with historical information on construction, operation and maintenance, should, among other things, ensure a high degree of planning security in order to minimise project risks. For direct use in planning and realisation, a higher data/model accuracy can be assumed than in the extended project environment.
  (This technical model can simultaneously form the basis for the planning of maintenance and repair work and provides important input for the planning.)
• Synergies between the technical models generate added value in planning and allow overdetermined control. The Retro BIM is the unique key between the models and can be used as a reference.
• Retro BIM gains additional benefit in early planning phases, when decisions are based on sound foundations and the influence on the project is big.

• A digital 3D georeferenced model that represents the existing structure according to a the requirements, but at least in its spatial geometry, being georeferenced and in a veritable manner (digital twin).
• All information contained should be centrally available, uniqe, structured and attributive in order to enable machine readability.
• The exchange of information via standardized interfaces and formats (e.g. IFC Standard) in order to avoid information losses.
• Creation of a common basis for all specific technical models.

Normative basics, standards, structures:

• Modeling standards (IFC standard, etc.)
• Client - specific lists for attributes, component catalogs, component-dependent definition (e.g. LoD (LoG, LoI))

Project-related basics / Input:
For the Retro BIM, any format-independent information available for the construction/project can be used.
A distinction must be made between the minimum requirements (geometry (georeferenced), the quality of the data (verified/not verified) and material) and all other specific information that generates added value for the information.

The following is a non-exhaustive list of possible basics:

• Data of the completed structure, such as 2D planning documents (digital or analogue) for shell, operating and safety equipment, etc.
• Point clouds from scanner images, etc. for the accuracy status of the data
• Information on drainage pipes (position, material, diameter, time of installation, etc.)
• Geometry data for pipes (scan images, spatial position of pipes, etc.)
• Track surveying, axle calculations, clearance analyses
• Data digital / analogue to subsoil investigation (drillings, tomographies, etc.)
• Information (reports, photo documentation, etc.) digital or analogue from condition surveys
• Information on buildings and operations
• Reports for quality controls, material inspections
• Information on monitoring measures (geodetic measurements, geotechnical measurements such as inclinometers, settlement measurements, sensor data such as water level, condition of sewerage system, etc.)
• Information on maintenance work / conversion work
• Geological maps
• Photogrammetry

This use case focuses on the creation and content of digital 3D models of existing infrastructure facilities. A distinction is made between minimum information (geometry (georeferenced), status of accuracy (verified/not verified), material) and specific information.

Based on the minimum information, the existing model can be enriched, extended, linked with further specific information (e.g. GI systems, databases). Any information that goes beyond the minimum requirements has a specific purpose. This is always subject to customer-specific requirements as well as the data structure ofcustomer-specific specifications (e.g. modeling guidelines for element accuracy, structuring for existing databases).

These specific technical models are the responsibility of the respective technical planners and are not part of this use case.