To meet increasing power needs of industrialized and urbanized consumers, power transmission companies are pressed to elevate the transmitted load in overhead power transmission lines. Increased loads impose thermal and mechanical stress, causing risks of transmission reliability. These risks can be mitigated by developing computational diagnostic tools for better scheduling of line inspections and maintenance. The thesis introduces a novel methodology for spatial analysis of conductor thermal aging based on historical weather data that can be performed at three different levels: point, line, and area. By utilizing this paradigm, the thermal state of an energized transmission line can be visualized, initiating a new dimension in thermal aging research. An advanced model for dynamic re-rating of existing transmission lines is introduced in the thesis to reduce the risk of sag. The performance of the thermal aging and re-rating models is examined using simulation data from the BC Hydro power transmission network.