Here is a post from Rich Owings. Make sure you have his site, www.gpstracklog.com book marked.
Free Garmin compatible maps of Europe
February 7, 2013 By
This is a frequent topic in my GPS land navigation class.
There are several factors that contribute to the accuracy of the signal data received and geographic coordinates displayed. Some of these factors include:
I understand that this may not be as "spot on" as some would like but overall the GPS will provide adequate information to reach the destination.
Depending on the model, I've found GPS compass information to be variable. Rather than use the GPS compass/electronic compass information exclusively I'll use my magnetic compass too.
To read the complete post go here.