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In the transmission line grounding system design, spatial and temporal/seasonal variations of tower footing impedance should be considered. However, in real life, only spatial variations are taken into account at the design stage, seasonal variations have not been included. In this article, seasonal variations of tower footing impedance of several types of transmission line grounding systems at Manitoba Hydro, Canada, are investigated through field measurements for a whole year. The industrial practice uses simulated tower footing impedances by the software current distribution, electromagnetic interference, grounding and soil structure analysis (CDEGS) in the design of transmission line grounding systems, however, the accuracy of simulated tower footing impedances as a critical design parameter has never been evaluated. In this article, this industrial practice is evaluated by comparing simulated and measured tower footing impedances for various transmission line grounding systems. The importance to use proper tower footing impedance for lightning conditions is also demonstrated. [ABSTRACT FROM AUTHOR]