THE ARPT HAD ONE ASPHALT RWY THAT WAS CLOSED & ANOTHER DIRT RWY THAT WAS MUDDY. THEREFORE, THE PLT ELECTED TO TAKEOFF ON THE PARKING RAMP WHICH WAS ORIENTED IN A NORTH/SOUTH DIRECTION & WAS 1700 FT LONG. THE PLT DID NOT CONSULT THE TAKEOFF CHARTS IN HIS OPERATING HANDBOOK. HE CONCLUDED THAT THERE WAS SUFFICIENT ROOM TO TAKEOFF SINCE THE ACFT NORMALLY USED ONLY 1500 FT AT HIS HOME ARPT IN OBERLIN, KS. HOWEVER, FOR TAKEOFF AT AURORA, CO, THE DENSITY ALT WAS APRX 7300 FT. ALSO, THE PLT USED SOFT FIELD PROCEDURES FOR TAKEOFF. SPECIFICALLY, BACK PRESSURE WAS USEDON THE CONTROL WHEEL FOR ALL OF THE TAKEOFF ROLL, THE NOSEWHEEL WAS LIFTED OFF EARLY & THE PLT TOOK OFF AT 60 KTS, THEN LOWERED THE NOSE TO STAY IN GROUND EFFECT. BEYOND THE END OF THE RAMP, THE ACFT HIT UNEVEN, RISING TERRAIN IN A WHEAT FIELD. FOR A SHORT FIELD TAKEOFF ON HARD SURFACE, NOSE-UP ELEVATOR INPUT IN NOT NORMALLY USED UNTIL ROTATION SPEED.