shining a flash light through a stretched out ball sack and having a friend trace the vains and other "landmarks" out on a piece of paper, then use a point on the map to decifer where you want to go for the night by comparing it to your local city map.
1."Lets do some sack mapping tonight to find out where we should kick it tonight."
2. Friend: "dude I dont know where to go tonight."
You: "me either, lets sack map it."