Catheterization of the heart is carried out using a framework formed by a plurality of electrically conducting wire loops. The wire loops are modeled as polygons, each subdivided into a plurality of triangles. The wire loops are exposed to magnetic fluxes at respective frequencies, and signals read from the loops. Theoretical magnetic fluxes in the polygons are computed as sums of theoretical magnetic fluxes in the triangles thereof, The location and orientation of the framework in the heart is determined by relating the computed theoretical magnetic fluxes to the signals.