Comparison of proper motion values derived in our 2010 disc paper of the central Arches field and in this study. Differences in the proper motion are shown individually in the right ascension (left) and declination (right) directions. The consistency between the blue and red samples reveals the similarity between the membership selection used to derive the central disc fraction in Stolte et al. (2010, red asterisks) and in this work (blue diamonds). Non-members are shown as black dots. The majority of stars in the common sample shows proper motion differences below 0.5 mas/yr, almost identical to the relative astrometric uncertainties in Arches Field 1 shown in Fig. 5. Faint stars (K_s< 16 mag) show larger motion differences, as expected from their higher astrometric uncertainties.