Sail plane pilot misjudges thermal activity for the day and flies beyond the point of safe return to the departure airport. An off airport landing ensues.

Narrative

I departed around XA35 in a glider. My intention was to fly around the area; with an overall objective of using a prominent tower as an intermediate turn point for the glider flight; where I would decide whether to fly further to the northwest and then come back. At around XB20 I reached the Tower; however; thermal activity was in deterioration and wind direction and strength were changing as well. I might have not reacted with the urgency I should have; since I was getting lower fast; however; when I realized that situation I headed east; first to try to reacquire altitude on a thermal and; second; to be closer to an area that could be more suitable for an off-airport landing in case altitude was not enough to return to an airport; since the area near the Tower is clearly flooded. While flying east; lift conditions deteriorated substantially turning into large areas of sinking air; therefore; rapidly increasing descent rate and leaving me no choice but to find the best possible area for an off field landing. At around XB45 after trying to re-establish a climb on a weak thermal; I decided to line out near a road and land the glider in an area that looked the most suitable; as per off-airport landing procedures. The glider landed and rolled about 170 FT before stopping. The surface of the field was very rough resulting in damage to the landing gear.