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In September 2003, Atel Transmission Ltd. in cooperation with ETRANS has started a pilot project for on-line thermal monitoring by an in-depth comparison of the state-of-the-art measurement techniques. The results of the field test will be used as the basis for future decisions to implement permanent on-line monitoring, in order to improve system operation. The direct or indirect conductor temperature measurement of different methods have been compared and correlated with line current measurements and high-quality meteorological data. The conductor temperature was recorded with a time resolution of 10 minutes using the following methods: Thermo-vision with the help of special cameras and subsequent transformation of colour to temperature; CAT 1 - mechanical tension sensors in combination with solar radiation and outside temperature measurements; SAW-(surface acoustic wave) sensors, a new approach of the University of Darmstadt; WAM-(wide area monitoring) line temperature monitoring approach - based on exact measurements of the active power losses. In addition to the overhead line conductor temperature measurements, the line sag was monitored for a short time period, too. These measurements give an indication of the final impact of conductor temperature variation. For line sag measurements the following two methods were analysed, the radar measurements and the laser measurements. Meteorological measurements were performed near the conductors at the same altitude. They consist of measurements of the ventilated ambient air temperature, solar radiation, 3-D wind speed and air humidity. These measurements have been compared with recordings from official meteorological stations, which collect their measurements at an altitude of 2 m (respectively 10 m for wind) above the ground. The results presented are collected from three geographical different locations (490 m altitude - moderate climate, 2130 m altitude - alpine climate and 190 m altitude - Mediterranean climate). As shown in the figures above, the 380 kV transmission line considered crosses three different climate zones within only 100 km. It starts in the northern part of Switzerland, traverses the Alps and ends in the southern part called Ticino. The test line is located in the middle of the UCTE power system.supplementary conductor heating. At the same time rain is a perfect cooling agent. One of the next project steps will be to calibrate corresponding line thermal calculation models with the acquired measurement data.