those are the middle numbers on your gps the 31 is the north and the last number is the est west. by middle numbers you dont need the 41 and the 82 only change every like 50 miles or so, so we just say the middle numbers So the 31.5 27.8 would be north 31.5 miles east 27.8 miles. so if you were fishing the 25/25 and some one said i'm killing them at the 30 /30 they would be 5 miles north of you and 5 miles west. hope that helps