Coastie

Although the earth hasn' t changed (much) our methods of showing it have. Our earth/globe has been modeled on various Datums (mathematical models) used to depict the roundness (or lack thereof) of the earth for a number of years. As our ability to more accurately depict this curvature has increased, the mapping agencies around the world have utilized this enhanced ability to correct certain assumptions made in earlier years. These assumptions have been proven for the most part to be very close to correct, however, there are some glaring exceptions.
On the east coast of the U.S for example, the difference between the maps drawn to NAD27 (North American Datum of 1927) and WGS 84 (World Geodetic Survey of 1984) have been proven to be very close, within less than a meter for the most part. In the South Western U.S. there have been more significant changes, differences in excess of several hundred feet (75-90 meters) have been noted and this is significant if you are drawing maps (WGS84 and NAD83 are essentially the same) In short, if you are using a map drawn to one datum and using as GPS, make sure the GPS is set to the same datum as your map. If you are using a map and a compass, ignore the whole thing and be sure you understand the deviation table referenced in the lower left hand corner of the map whne you are using your magnetic compass as well as understanding how to read a map in the first place.