  Cross-correlation signals are recorded from fluorescence photons scattered in
free space off a trapped ion structure. The analysis of the signal allows for
unambiguously revealing the spatial frequency, thus the distance, as well as
the spatial alignment of the ions. For the case of two ions we obtain from the
cross-correlations a spatial frequency $f_\text{spatial}=1490 \pm 2_{stat.}\pm
8_{syst.}\,\text{rad}^{-1}$, where the statistical uncertainty improves with
the integrated number of correlation events as $N^{-0.51\pm0.06}$. We
independently determine the spatial frequency to be $1494\pm
11\,\text{rad}^{-1}$, proving excellent agreement. Expanding our method to the
case of three ions, we demonstrate its functionality for two-dimensional arrays
of emitters of indistinguishable photons, serving as a model system to yield
structural information where direct imaging techniques fail.
