At the posted coordinates you will find a map of Michigan. Below the map, you will find a plaque with information about the rest area. On the plaque, you will need to add up the distances from this rest area to the following places: Michigan City, IN, Gary, IN and Chicago. After you have that number, add 200 and those are the final 3 digits to the West coordinates.
Coordinates are follows:
N 42° 10.880
W 086° 14.XXX
Formula for solving XXX: Distance to Michigan City + Distance to Gary + Distance to Chicago + 200.
