After landing at CHO at night; reporter was instructed to taxi to the ramp. Assuming that Taxiway C continued to the ramp the reporter taxied straight ahead and down a grass slope through a dip then onto the ramp. Propeller struck the grass during the excursion.

Narrative

Landed on Runway 3 at CHO; exited on Taxiway C. Time was 25-30 minutes after sunset; thus not officially 'night'; but it was very dark. CHO is in a mountain valley; and it gets dark quickly after sunset. I was unable to make out any features on the ground. Stopped at 'hold short' line on taxiway C. Tower - which was also serving as Ground Control - then advised; 'Taxi to ramp.' No further communications from Tower. Sitting on the taxiway; I saw the lighted FBO facility directly in front of me; and it appeared that the ramp extended to my position. However; at the end of Taxiway C there is actually a strip of grass separating it from the FBO ramp area; which I was not aware of; and this grass has a downward slope towards the ramp; with a slight dip at the end. (At the next exit from the runway; Taxiway D; by comparison; there is no such grass; and the ramp extends directly to the taxiway; I was not aware of this difference). I slowly taxied forward and ran off the taxiway and onto the grass. By the time I realized what was happening; the aircraft was sliding down the slope and across the dip at the bottom. Brakes were to no avail. As the aircraft went across the dip; the prop contacted the grass; and the tail dragged on the grass. The aircraft stopped a few feet forward; on the asphalt ramp.This mishap could have been avoided if either (1) I had studied the airport diagram more closely and noticed the grass strip between Taxiway C and the ramp; or (2) Tower had advised me to turn left or right at the end of Taxiway C; onto Taxiway A; instead of just saying 'taxi to ramp'. Talking with the Tower Controller afterwards; it became apparent that he was not following my movement on the field; for he remarked that he was not aware of the mishap until emergency vehicles came on the scene; and he had thought they were engaged in a practice exercise of some sort.