How to get the velocity from GPS?

[vincentsiau]

Hi, I wanted to get the speed of my vehicle using GPS.
I found that the NMEA sentences $GPVTG shows the speed over ground in know and in km/h.
Does the speed over ground in km/h means the speed of my vehicle if I put the GPS device in my vehicle?How is the accuracy?
If that is not the speed of my vehicle, is there any way to get or calculate the speed of my vehicle?

Thanks,
Vincent

 

[Nuvi-Nebie]

Yes, ground speed is the speed you travel across the ground when travelling in a car or train etc, you would have different 'speeds' if you was in an aeroplane, or boat, because the air (or water) can also have a speed, e.g. if you are flying in a plane with an airspeed of 100 km/h, but the wind is blowing against you at 20km/h your ground speed would only be 80km/h. Details on $GPVTG are :-

VTG - Velocity made good. The gps receiver may use the LC prefix instead of GP if it is emulating Loran output.

$GPVTG,054.7,T,034.4,M,005.5,N,010.2,K*48

where:
VTG Track made good and ground speed
054.7,T True track made good (degrees)
034.4,M Magnetic track made good
005.5,N Ground speed, knots
010.2,K Ground speed, Kilometers per hour
*48 Checksum