Some (perhaps most?) GPS receivers have this kind of algorithm (hold last heading below 2-5 km/hr) implemented on-board. This works well because it tends to show the last meaningful direction, unless you are in a sharp turn as you come to a stop.

The nice thing about GPS velocity measurement is that is a direct computation from the doppler shift of the satellite signals, so it doesn't have the noise contribution you get from differentiating position, as was the case for older radio navigation systems. Three dimensional velocity error is, therefore, independent of vehicle speed for a GPS system, typically around 0.2 m/sec or better for commercial equipment.

Definitly appropriate for calibrating your speedometer!

Lynn