The accurate transmission line parameters are important in various power system analyses, while potential uncertainty in their values affects the accuracy of control center applications and compromises the selectivity of the protection systems. The transmission line parameters are usually calculated based on manufacturers data, ignoring environmental factors (e.g. ambient temperature) which affect the accuracy of the calculated parameters. Thus, the systematic refinement of the line parameters can be very beneficial for the situational awareness of the power system operators. In this paper, the calculation of the positive sequence parameters of a transmission line of the Cyprus power system through real synchronized phasor measurements is presented. The seasonal variation of the transmission line parameters is analyzed by calculating the parameters during different periods of the year, while the impact of the instrument transformers static error is demonstrated in this real case study.