Shower centre determination in a high granularity digital sampling calorimeter

The precise measurement of photon or electron impact position is the prerequisite to reconstruct the origin of two photons events from decay of {\pi}0 and {\eta} mesons or the di-electron channel from quarkonia decay. To facilitate such a measurement, an electromagnetic calorimeter prototype using Monolithic Active Pixel Sensors (MAPS) was build and tested. The 30 $\times$ 30 m$^2$ granularity coupled with an optimized centre of gravity with power law weight method dedicatedly developed for the particle counting digital calorimeter, makes precise reconstruction of impact position of electromagnetic showers to be straightforward. Parameters of the method are optimized and the performance is verified with the GEANT based simulation and the experimental data which were collected at DESY and CERN SPS. The accuracy of the reconstruction of electron or photon impact position can be better than 30 $\mu$m in a broad range of energies.