A polycrystalline recrystallized α-Fe specimen was subjected to monotonic tension up to 2 % strain. Local strain was measured all over the width of the specimen thanks to in-situ optical-based Digital Image Correlation with adequate spatial and temporal resolution. With additional knowledge of local crystallographic orientation thanks to ebsd scan before the test, strain heterogeneities and localizations were analyzed in depth. A downscaling global to local strategy was employed. At the local scale, the displacement gradient tensor was projected onto every potential slip system for fine investigation of slip activity. The results indicate that the strain structure develops at a scale larger than the grain’s and leads to strain localizations around grain boundaries. The analysis of the build-up of the plastic deformation pattern has been differentiated from the one of the occurrence of strain localizations. Firstly, no correlation is found between the plastic structure and grain size but a weak one is found with grain orientation; the major influence of grain interactions has been discussed. Secondly, strain localizations are observed when several slip systems are activated simultaneously in a reduced area.