Why Your BIM Model is Floating in Space (and How to Fix It)

If you work in BIM (Building Information Modeling) or GIS (Geographic Information Systems), you are likely familiar with the “floating building” problem. You open a beautifully detailed IFC model, but instead of sitting neatly on a specific parcel of land, it’s floating in a void, anchored only by a generic 0,0,0 coordinate.

Bridging the gap between the infinite precision of BIM and the geodetic reality of GIS is one of the industry’s most persistent headaches. A journal paper by Clemen Christian and Görne Hendrik tackles this head-on by proposing a standard metric: the Level of Georeferencing.

They provide a ladder of quality from simple addresses to full coordinate reference systems. This allows stakeholders to specify exactly how “geo” BIM data needs to be.

Here is a breakdown of the ladder and what they mean for your projects.

The framework is divided into tiers. The higher the number, the higher the quality of the georeferencing. Crucially, these levels are independent; a Level 50 model doesn’t automatically imply it has Level 10 data, though ideally, it might.

• The Concept: The simplest way to say where a building is located is to write the address on it.

• The Tech: This level uses the IfcPostalAddress entity. It includes standard fields like town, postal code, and address lines.

• The Utility: While not mathematically precise enough for surveying, it is human-readable and allows for basic integration with surrounding city models or GIS data.

• The Concept: This places a single reference point (Lat/Long) on the map.

• The Tech: It utilizes RefLatitude, RefLongitude, and RefElevation attributes within the IfcSite entity.

• The Utility: This anchors the model to a spot on Earth (usually WGS84). However, it generally lacks rotation parameters, meaning you know where the building is, but not which way it is facing.

• The Concept: This defines the location of the site relative to the world origin using a “3+1” transformation: 3 coordinates for position (X, Y, Z) and 1 vector for rotation.

• The Tech: It uses the IfcLocalPlacement of the IfcSite. The placement is relative to the “world” (meaning the attribute PlacementRelTo is empty).

• The Utility: This allows for a specific Cartesian translation and a rotation to True North. It is a massive step up from Level 20 because it orients the building correctly.

• The Concept: Instead of just placing the Site, this level configures the geometric context of the entire Project.

• The Tech: This relies on IfcGeometricRepresentationContext within IfcProject. It uses the WorldCoordinateSystem for location and TrueNorth to define the Y-axis rotation relative to global North.

• The Utility: This is a common method for handling rotation (True North) and coordinates at the project level, ensuring all sub-models inherit the correct orientation.

• The Concept: This is the highest standard for defining coordinate systems, but it requires IFC4.

• The Tech: It introduces specific entities like IfcMapConversion and IfcProjectedCRS.

  • Map Conversion: Handles Eastings, Northings, Orthogonal Height, Scale, and Axis rotation.

  • Projected CRS: Allows you to specify the exact EPSG code (e.g., EPSG:31467).

• The Utility: This explicitly links the BIM internal coordinates to a national grid or geodetic datum, handling scale distortion and elevation offsets professionally.

• The Concept: This level addresses the reality of construction surveying, where identical “control points” are measured in both the BIM local system and the national grid.

• The Problem: Currently, standard IFC entities cannot easily store this “three-step transformation” (BIM -> Engineering Grid -> Geodetic Grid).

• The Solution: Some suggests this must be handled via custom property sets. Others feel this is not up to IFC. bSDD could be a solution to ‘expand’ the entities of IFC. A more long term solutions seems to be the intended modularisation in IFC 5.

For BIM managers, this framework provides a vocabulary for the BIM Execution Plan (BEP). Instead of vaguely asking for “georeferenced models,” you can require “GeoRef 50 compliance.”

For GIS experts, it helps manage expectations. It clarifies that while a BIM model might have coordinates (Level 20), it might lack the projection metadata (Level 50) needed for high-precision analysis.

Next Step: Check your current standard BIM template. Are you exporting IfcMapConversion data? If you are using IFC 4 or 4.3, you might be closer to Level 50 than you think.