There are several reasons why comparing coordinates from different sources may give you a location error. The bottom line is that you need to ensure that the datums and coordinate formats of your sources agree.
1. Datum. The most common datum is NAD83 (or WGS84), and most GPS units are set to it by default. MapPass uses the NAD83 datum. This usually is not a problem, but if you are using a coordinate from a source using NAD27 datum, your location may be off by several hundred yards or more.
2. Coordinate format. Latitude and Longitude coordinates are shown in 3 common formats:
Decimal Degrees: 39.6891°
Degrees and Minutes: 39° 41.346'
Degrees, Minutes, and Seconds: 39° 41' 20.76"
All of the above numbers represent the same coordinate in a different format. You need to be sure that your GPS or other coordinate source is set to the same format as the MapPass.
The MapPass default format is Decimal Degrees. GPS devices generally default to Degrees and Minutes. To resolve the discrepancy, you can go to the "Change Coordinate System and Units" link below the MapPass map and change it to Degrees/Minutes.
You can also change the coordinate system on your GPS. The way to do this varies by GPS unit, but the formats above are often represented in the settings as DD.DDDDD, DD MM.MMMM, and DD MM SS.SS.