When obtaining a position, it is important to note the number of satellites in view at the time of the reading. In general, if there are fewer than four satellites in view, the accuracy of the reading will be compromised. When accuracy is critical, ensure that the GPS unit has a clear view of the sky, free of any obstructions. Even large buildings on one or more sides of the GPS unit will skew the signal coming from the satellites.
Also, although USGS topographic maps are the finest and most accurate available, they are not perfect. USGS 7.5-minute quads adhere to "National Map Accuracy Standards." In a nutshell, this means that a 1:24,000 scale map is accurate within 40 feet. Additional information can be obtained from the USGS website: www.USGS.gov
Until May 2000, GPS readings could have been off as much as 100 meters (110 yards or 328 feet). Although the government reserves the right to reinstate this 'Selective Availability' during times of military campaigns, civilians can now enjoy the full pin-point accuracy that GPS provides.
Other factors that can give the impression of a false location include the use of different datums, and/or the use of a different coordinate format.
Generally speaking, GPSs report their coordinates in WGS84 datum. If your coordinates are off a little bit, you may be looking at a datum shift. Set the datum in Terrain Navigator (in the Coordinate Preferences) to match that of the GPS before entering coordinates. (If you are not sure the datum of your coordinates, WGS84 is usually a safe bet.)
Also, be sure that the format of the coordiates match exactly. Latitude/Longitude can be expressed in Degrees Minutes Seconds (Dº M' S"), Degrees Minutes (Dº M.M'), and Decimal Degrees (D.Dº). Again, be sure that the setting in your GPS matches the Coordinate Preference selected in Terrain Navigator.