I flew locally in my glider, the software logs the position and height (every 5 secs S&L,every 1 sec turning) . Afterwards I loaded the file into flight analysis software, which shows the flight over a background similar to Google Earth, and which showed the take off position as 90 m to the NE of the actual(in fact outside the airfield boundary). After adjusting to airfield QFE the trace showed me continuing to climb for some time, to 1750 ft, after releasing the winch cable at 1150 ft. This did not actually happen, I had started to descend. The log also showed me hill soaring at 1200/1400 ft airfield QFE, when in fact I was at 600/800 ft. I have been using this setup for quite a long time, and similar equipment for 6/7 years, and never had errors of anything like this magnitude.I cannot be clear at this stage if it was equipment malfunction or not, but it certainly now reports a correct position.