The airplane taxied off the hard surface into the soft grass, coming to rest about 50 feet beyond the edge of the hard surface taxiway. The captain stated that after turning south on NK, he taxied the airplane towards the IAB terminal, and saw what appeared to be a hard surface ramp to the left and right of the airplane and he proceeded to make a left turn believing that he was now taxiing on the IAB ramp. The captain reported that the airport diagram on the Jeppesen charts showed the intersection of NK and the IAB ramp to be a ‘T’ or 90 degree turn, but in reality, the intersection is constructed at a 45 degree angle to the left of taxiway NK which joins the IAB ramp. The pilot further stated that no lights were available to outline the edge of the ramp area. He added that he continued to taxi in an east direction, believing that he was on the IAB ramp. According to the airport’s operations personnel, the taxiway is properly marked with a solid center line and the area is bordered by a double yellow line delineating the edge of the taxiway. Additionally, at the time of the accident, recessed taxiway lights were in place to further delineate the edge of the taxiway. Probable Cause: The pilot’s failure to follow the proper taxi route while taxiing back to the terminal. A factor was the glare radiating from the airport’s terminal area.