1. A system, comprising: (a) an electrical interface; and (b) a processor, configured: (i) to receive, via the electrical interface, a plurality of signals from a probe, as the probe is placed at respective locations on a body of a subject, wherein the probe is positioned at respective orientation axes corresponding to the respective locations, such that each location has a corresponding probe orientation axis, (ii) to derive, from the signals, a set of points that correspond to the respective locations and associated probe orientation axes, (iii) to register the set of points with a three-dimensional image of the body of the subject, wherein the three-dimensional image comprises a set of voxels, each voxel having an associated surface normal, wherein the processor is configured to register the points by: (A) identifying, for each point of the points, one or more voxels of the image whose respective surface normals are oriented within a given angle of the probe orientation axis corresponding to the location to which the point corresponds, (B) selecting, for each of the points, a voxel from the one or more identified voxels, and (C) computing a transformation that aligns each point of the points with a selected one of the voxels that were identified for the point, and (iv) to track, subsequently, based on the registering, a location of an intrabody tool, in a coordinate system of an imaging device that was used to acquire the image.