The purpose of the flight was to aero tow a glider from a paddock where it had made an outlanding. Prior to takeoff the pilot maesured the paddock twice by pacing. The paddock was wedge shaped with powerlines running along the two longer boundaries. There was also a powerline spanning the paddock towards the narrow end of the wedge. Despite walking under this powerline while pacing the paddock, the pilot did not see it. Takeoff was into the northwest which provided the longest take-off run and was also into a 5-7 kt wind. After takeoff the pilot held the tug down to gain speed. He then saw the powerline but too late to avoid it. After impact with the powerline the tug hit the ground and was substantially damaged. The pilot said that in the take-off direction, the powerline was below the horizon against a background of hills which made it almost impossible to see.