Meant to add ...

Although some systems say that they are accurate to 3m you can not always get this resolution. If you are working from someone else's GPS reading and they only had an accuracy of 10m when they took the reading and you only have a similar accuracy you could be upto 20m from the point they were at when they took their reading. This is fine if you're heading for a tomb or stone circle, but not so great if you're looking for a small rock art panel in the middle of a hillside covered in gorse. Then patience comes in - something I lack in enormous quantities.