Figure 1.

Two‐dimensional schematic showing how more sampling directions reduce the ambiguity in the back projection. A: Two directions with three markers could represent nine locations. B: A third direction removes the ambiguity and uniquely identifies the probes. C: A slightly different configuration, however, is still ambiguous. D: A fourth direction resolves the issue. In practice, a statistical approach is needed, as described in the text. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]