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Properly Locating and Georeferencing Models

Learn when a local coordinate system is sufficient, when true georeferencing is appropriate, and how to ensure that your domain models are correctly overlaid.

Recommendations for Model Location, Coordinate Systems and Georeferencing

When several discipline models from different planning participants are combined into an overall model in big®, their coordinates and reference points must align with one another. This article explains when a local coordinate system is sufficient, when true georeferencing makes sense, and which information should be defined consistently across the project.

Why is model positioning important?

Whether discipline models are correctly aligned on top of one another depends on several factors. These include, in particular:

  • the planning software being used
  • the IFC version selected
  • the export settings of the individual discipline models
  • the coordinate and elevation system being used
  • how the respective viewer interprets the positioning information

Differences between models are therefore not automatically a planning error. They are often caused by different settings or by reference systems that have not been defined consistently.

The earlier model positioning is coordinated and documented within the project, the lower the subsequent coordination effort will be.

Which approach is right for your project?

In principle, there are two possible approaches:

Project type Recommended approach
Single, connected building Local coordinate system
Campus, district, larger external works or infrastructure project Georeferencing using real-world coordinates

Both approaches are valid. The key requirement is that all project participants use the same specifications.

Single buildings: local coordinate system

For conventional building projects involving a single, connected building, a simple local coordinate system has proven effective.

Recommended settings

  • Use several clearly identifiable reference points or reference objects. In practice, three control points are generally useful.
  • Place one reference point at the local origin (0, 0, 0).
  • Export the model based on the local project base point — in Revit, for example, using the Project Base Point rather than Shared Coordinates.
  • Align the model as orthogonally as possible with the coordinate system.

Advantages of this approach

  • Discipline models can generally be combined without having to move them afterwards.
  • Drawing calibration and model coordination are simplified.
  • An officially surveyed reference point is not required at the beginning.
  • The coordination effort between project participants is reduced.

The local approach is therefore a good starting point, provided there are no project-specific reasons or existing requirements from the client or participating design offices that would suggest otherwise.

Campus, district and infrastructure projects: true georeferencing

For projects involving multiple buildings, larger external areas or infrastructure, a purely local coordinate system often reaches its limits. In these cases, referencing the project to real-world, official coordinates is generally more appropriate.

Define a common reference system

Agree at an early stage which geodetic reference system and map projection will be used. In Germany and large parts of Europe, for example, ETRS89 in combination with UTM is frequently used. Depending on the country, project or client requirements, other systems may also be appropriate.

The most important point is that all participants use the same reference system.

Define the Elevation Reference Separately

The horizontal position and elevation reference must be considered separately. Therefore, also define which elevation system the models are based on, for example Normalhöhennull (NHN).

Use Official Survey Control Points

Where possible, reference the project to actual surveyed control points. Freely selected reference points often make subsequent coordination with the following more difficult:

  • Existing surveys
  • Geospatial data
  • External works
  • Utility and infrastructure models
  • Adjacent projects

Use the Intended IFC4 Standard Approach

From IFC4 onwards, dedicated data structures are available for transferring georeferencing information. These include, in particular:

  • IfcProjectedCRS for information about the coordinate reference system being used
  • IfcMapConversion for converting between the local model coordinate system and the higher-level map coordinate system

Where possible, we recommend using this standardized approach instead of older, proprietary, or software-specific additional fields. This increases the likelihood that different software applications will interpret the coordinates consistently.

Document Requirements Centrally

At a minimum, centrally document the following information, for example in the BIM Execution Plan:

  • Coordinate reference system used
  • Map projection used
  • Elevation reference
  • Location and designation of control points
  • Local model origin
  • North direction or project rotation
  • Agreed IFC version
  • Mandatory export method and export settings

This ensures that project participants who join the project at a later stage can also access the same requirements.

General Recommendations

Regardless of the approach selected, the following practices have proven effective:

  1. Standardize the IFC version
    Wherever possible, use the same IFC version for all discipline models. Location information may be stored differently depending on the IFC version.
  2. Coordinate export settings
    Communicate the agreed export method transparently to all project participants and document it in writing.
  3. Check reference points
    Whenever models are exchanged, verify that the defined reference points still align correctly.
  4. Prefer orthogonal alignment
    Orthogonal alignment often simplifies drawing calibration. However, for georeferenced projects with an actual site boundary, this is not always possible or appropriate.
  5. Cross-check the model at an early stage
    Open the IFC files in a second, independent viewer for testing. This makes it possible to identify at an early stage whether the models are fundamentally positioned correctly relative to one another or whether a setting is being interpreted differently by a particular platform.

Checking the Georeferencing of an IFC File

With the free web-based IFC Georeferencer from buildingSMART, you can check which location information is stored in an IFC file. The tool displays the relevant geospatial information in an easy-to-understand way and, if required, allows you to georeference the file directly in the browser.

According to the provider, processing takes place client-side in the browser; the IFC file is not uploaded to a server in the process.

Open IFC Georeferencer

In Summary

  • For individual, self-contained buildings, a simple local coordinate system is usually sufficient.
  • For campus, district, and infrastructure projects, true georeferencing with surveyed reference points is recommended.
  • The coordinate reference system and elevation reference must be clearly defined.
  • From IFC4 onwards, the designated structures IfcProjectedCRS and IfcMapConversion should be used wherever possible.
  • Consistent IFC and export settings help avoid unnecessary coordination effort.
  • All requirements should be defined at an early stage and documented centrally.

Are you unsure which approach is suitable for your project? Your Kaulquappe project team will be happy to support you in selecting and checking the appropriate model positioning.