Happy 18th Birthday! sorry if i'm late.
gps co-ordinations tend to be quite long (assuming you'll have it done in one line)
so i would suggest tattooing it in a place which won't stretch as much, or change when you move.
An example of tattoos changing when you move is a long tattoo on your forearm running from your elbow down. When you turn your hand the tattoo tends to twist from being straight into being slanted.
I would suggest your foot, wrist, back or Angelina Jolie's placement of the shoulder.
Remember, these are only suggestions, it's down to what you like in the end ;)