Why do GPSy and my GPS unit disagree slightly about how fast Im going, how far I need to travel to the next waypoint, or my altitude?
GPSy automatically converts all distance/speed units to your preferred units (set in the Display Preferences panel). However, since the GPS unit may introduce rounding errors when sending the NMEA sentence to the computer, you may not see exactly the same data on the GPS screen as GPSy. For example, when a Magellan GPS unit displays an altitude of 42 feet on its LCD display, it sends an altitude of 14.0 meters to GPSy via NMEA. When GPSy recalculates 14.0 meters into feet, it comes out to a different altitude than the original. Usually these rounding errors are minimal and are due to the NMEA sentence not having the full resolution of either GPSy or the NMEA device. All GPSy conversions are done using 6 digits of precision in the conversion factor and standard C double precision floating point.
