Wot Justinmg said. The ground database, like the marine database, may not have been resurveyed for a hundred years, furthermore, air charts don't include the reliability diagrams that would alert you to the potential inaccuracy like a marine chart does.
Your Garmin should be accurate to a few yards in position (but not altitude, thanks to WGS 84). I have experienced one satellite failure (thankfully at sea) where the GPS transported me instantly thousands of miles to South East Asia until it was gagged.