IFC Element Positioning and Labeling with Respect to Axes: A Reproducible Methodology Using xBIM

This paper presents a unified algorithm for the automatic labeling of BIM elements on floor plans with respect to IFC axis grids. Axes from IfcGrid/IfcGridAxis (letters, numbers, or mixed tags such as [LT-3] or [G'']) are grouped by orientation, assigned a canonical normal, and indexed using an order scalar derived from the projection of axis midpoints. With this onedimensional indexing, the algorithm identifies point elements (axis intersections), linear elements (nearest host plus orthogonal range), and area elements (ranges in two axis families) without user intervention. The system automatically selects the classification path (area → line → point) according to element size. The geometric core relies on projection and bracketing, selecting bounding axes by applying tolerance and proximity criteria. Robustness is achieved through automatic flipping of normal vectors, endpoint selection by 1D distance, and canonical ascending label formatting ([A–B], [9–11], [LT-2–LT-3]). The implementation in .NET (VB) employs xBIM for IFC parsing and footprint extraction. Evaluation on synthetic and real IFC datasets measures label accuracy, endpoint accuracy, and projection error, with ablation studies on tolerance and auto-flip. Limitations include curvilinear grids, nearly parallel families, and complex footprints. The approach is simple, interpretable, and reproducible, supporting traceability in BIM workflows.