Abstract: A method to identify the surface of solid models immersed in fluid flows is devised that examines the spatial distribution of flow tracers. The fluid–solid interface is associated with the distance from the center of a circle to the centroid of the tracers ensemble captured within it. The theoretical foundation of the method is presented for 2D planar interfaces in the limit of a continuous tracer distribution. The discrete regime is analyzed, yielding the uncertainty of this estimator. Also the errors resulting from curved interfaces are discussed. The method's working principle is illustrated using synthetic data of a 2D cambered airfoil, showing that one of the limitations is the treatment of an object thinner than the search circle diameter. The method is readily adapted to 3D and applied to the 3D PTV data of the flow around a juncture. The surface is reconstructed within the expected uncertainty, and specific limitations, such as the smoothing of sharp edges is observed. Graphic abstract: [Figure not available: see fulltext.].