Forming limit curve (FLC) is a useful tool to characterize the formability of sheet metals. It can be used as a failure criterion in a laboratory setting or computer simulation for forming/stamping studies and in actual production in a press shop to assess the formability severity. Digital image correlation (DIC) technique has been proved as a potent tool to determine the FLC of sheet metals. Despite of its rising popularity, one of the major technical challenges using the DIC to generate FLC is to accurately pinpoint the onset of localized necking based on DIC data analysis. In addition to the commonly applied ISO 12004-2 standard, a plethora of DIC data analysis approaches have been developed. In this study, five different approaches have been practiced to determine the limit strains at the onset of localized necking, including: ISO12004-2 standard, second derivative, gliding correlation coefficient, linear best fit, and curvature assisted necking zone methods. The formability of a 980GEN3 sheet steel was studied under plane strain condition in this work using the Marciniak cup test coupled with a 3D DIC system. The resulting forming limits determined by different methods were compared. Strengths and limitations of each method were discussed. In addition, the conventional finger-touch method was excised using specimens with perceivable localized necks. Strain measurements associated with localized necks detected by the finger-touch method were used as the basis to evaluate the effectiveness of selected approaches. Initial suggestions were made based on this case study regarding the harmonization in DIC data analysis methods for the FLC determination.