I am afraid that this is either too specific and\or that this is in the wrong forum (if the latter is the case, if moderators could change this, I'd appreciate :). I'm trying to do an application that uses the GPS data that comes from NMEA files. On a particular sentence (RMC) there is a value indicating "speed". After looking about it, I found a place that described this speed as in "Speed over ground, Knots". I have two doubts regarding this value: Exactly what does it mean? Is it the speed having the ground as a reference, or a projection on the ground, making it a 2D speed (not caring about differences in the altitude.)? Also, how accurate is it? How is it calculated?